Using the ActionBar in 1.8 MC clients (currently written for the Spigot protocol hack). Explanation here: http://www.spigotmc.org/threads/news-bar-over-the-hotbar.30394/#post-345167

ActionBarExample.java

package net.darkseraphim.actionbar;

import java.lang.reflect.Field;
import java.lang.reflect.Modifier;
import net.minecraft.server.v1_7_R4.EnumProtocol;
import net.minecraft.server.v1_7_R4.NetworkManager;
import net.minecraft.server.v1_7_R4.Packet;
import net.minecraft.server.v1_7_R4.PacketPlayOutChat;
import net.minecraft.util.com.google.common.collect.BiMap;
import net.minecraft.util.io.netty.channel.Channel;
import org.bukkit.Bukkit;
import org.bukkit.craftbukkit.v1_7_R4.entity.CraftPlayer;
import org.bukkit.entity.Player;
import org.bukkit.plugin.java.JavaPlugin;


public class ActionBarExample extends JavaPlugin
{

    private BiMap map;

    private ProxyHashBiMap<Integer, Class> special;

    @Override
    public void onEnable()
    {
        overrideI();
        injectCustomPacket(PacketPlayOutChat.class, PacketPlayOutActionBar.class);
        refreshPlayers(true);
    }
    
    @Override
    public void onDisable()
    {
        refreshPlayers(false);
        ejectCustomPacket(PacketPlayOutChat.class, PacketPlayOutActionBar.class);
        returnI();
    }

    private void overrideI()
    {
        try
        {
            Field field = EnumProtocol.class.getDeclaredField("i");
            field.setAccessible(true);
            this.map = (BiMap) field.get(EnumProtocol.PLAY);
            special = new ProxyHashBiMap<Integer, Class>(map);
            set(field, EnumProtocol.PLAY, special);
        }
        catch(Exception ex)
        {
            ex.printStackTrace();
            throw new ExceptionInInitializerError("Nope, not allowed.");
        }
    }

    private void set(Field f, Object o, Object v) throws NoSuchFieldException, IllegalAccessException
    {
        boolean accessible = f.isAccessible();
        f.setAccessible(true);
        if ((f.getModifiers() & Modifier.FINAL) != 0)
        {
            set(Field.class.getDeclaredField("modifiers"), f, f.getModifiers() & ~Modifier.FINAL);
        }
        f.set(o, v);
    }

    public <T extends Packet> void injectCustomPacket(Class<T> mc, Class<? extends T> custom)
    {
        special.inverse().injectSpecial(PacketPlayOutActionBar.class, PacketPlayOutChat.class);
        try
        {
            Field field = EnumProtocol.class.getDeclaredField("f");
            field.setAccessible(true);
            ((Map)field.get(null)).put(custom, EnumProtocol.PLAY);
        }
        catch (NoSuchFieldException ex)
        {
            ex.printStackTrace();
        }
        catch (IllegalAccessException ex)
        {
            ex.printStackTrace();
        }
    }

    public <T extends Packet> void ejectCustomPacket(Class<? extends T> custom)
    {
        special.inverse().ejectSpecial(custom);
        //Validate.notNull(id, "You can only eject existing packets");
        try
        {
            Field field = EnumProtocol.class.getDeclaredField("f");
            field.setAccessible(true);
            ((Map)field.get(null)).remove(custom);
        }
        catch (NoSuchFieldException ex)
        {
            ex.printStackTrace();
        }
        catch (IllegalAccessException ex)
        {
            ex.printStackTrace();
        }
    }
    
    private void returnI()
    {
        try
        {
            Field field = EnumProtocol.class.getDeclaredField("i");
            field.setAccessible(true);
            set(field, EnumProtocol.PLAY, this.map);
        }
        catch(Exception ex)
        {
            ex.printStackTrace();
        }
    }
    
    private void refreshPlayers(boolean enable)
    {
        try
        {
            Field channelField = NetworkManager.class.getDeclaredField("m");
            channelField.setAccessible(true);
            Channel channel;
            for (Player player : Bukkit.getOnlinePlayers())
            {
                channel = (Channel) channelField.get(((CraftPlayer) player).getHandle().playerConnection.networkManager);
                channel.attr(NetworkManager.f).set(enable ? this.special : this.map);
            }
        }
        catch(Exception ex)
        {
            ex.printStackTrace();
        }
    }
}

PacketPlayOutActionBar.java

package net.darkseraphim.testmodule;

import java.io.IOException;

import com.comphenix.protocol.events.PacketEvent;
import net.minecraft.server.v1_7_R4.*;

import net.md_5.bungee.api.chat.BaseComponent;
import net.md_5.bungee.api.chat.TextComponent;
import net.md_5.bungee.chat.ComponentSerializer;

import org.bukkit.Bukkit;
import org.bukkit.ChatColor;
import org.bukkit.craftbukkit.v1_7_R4.entity.CraftPlayer;
import org.bukkit.craftbukkit.v1_7_R4.util.CraftChatMessage;
import org.bukkit.entity.Player;


/**
 * @author DarkSeraphim
 */
public class PacketPlayOutActionBar extends PacketPlayOutChat
{
    public String json;

    /**
     * Creates an action bar message
     * @param message The message to display
     */
    public PacketPlayOutActionBar(String message)
    {
        super();
        this.json = "{\"text\":\""+message+"\"}";
    }

    /**
     * When using md_5's chat API
     * @param components Components of which the chat consists of
     * @deprecated Doesn't support colours and formatting
     */
    @Deprecated
    public PacketPlayOutActionBar(BaseComponent...components)
    {
        super();
        this.json = ComponentSerializer.toString(components);
    }

    public void a(PacketDataSerializer packetdataserializer) throws IOException
    {
        this.json = ChatSerializer.a(ChatSerializer.a(packetdataserializer.c(32767)));
    }

    public void b(PacketDataSerializer packetdataserializer) throws IOException
    {
        packetdataserializer.a(this.json);
        packetdataserializer.writeByte(0x2);
    }

    public void a(PacketPlayOutListener packetplayoutlistener)
    {
        packetplayoutlistener.a(this);
    }

    public String b()
    {
        return String.format("actionbar='%s'", new Object[] { this.json });
    }

    public void handle(PacketListener packetlistener)
    {
        a((PacketPlayOutListener)packetlistener);
    }

    // Added some helper methods
    public void send(Player player)
    {
        if(((CraftPlayer)player).getHandle().playerConnection.networkManager.getVersion() < 47)
            return;

        ((CraftPlayer)player).getHandle().playerConnection.sendPacket(this);
    }

    public void broadcast()
    {
        for(Player player : Bukkit.getOnlinePlayers())
        {
            send(player);
        }
    }
}

ProxyHashBiMap

package net.darkseraphim.testmodule;

import net.minecraft.server.v1_7_R4.PacketPlayOutChat;
import net.minecraft.util.com.google.common.collect.BiMap;
import net.minecraft.util.com.google.common.collect.HashBiMap;

import javax.annotation.Nullable;
import java.util.HashMap;
import java.util.Map;
import java.util.Set;

/**
 * @author DarkSeraphim
 */
public class ProxyHashBiMap<K, V> implements BiMap<K, V>
{

    private BiMap<K, V> delegate;

    private ProxyHashBiMap<V, K> inverse;

    // This will proxy
    protected Map<K, K> proxy = new HashMap<K, K>(1);

    public ProxyHashBiMap(BiMap<K, V> map)
    {
        this.delegate = map;
        this.inverse = new Inverse(this, map.inverse(), map);
    }

    private ProxyHashBiMap(BiMap<K, V> map, BiMap<V, K> inversed)
    {
        this.delegate = map;
        this.inverse = null;
    }

    @Override
    public int size()
    {
        return this.delegate.size();
    }

    @Override
    public boolean isEmpty()
    {
        return this.delegate.isEmpty();
    }

    @Override
    public boolean containsKey(Object key)
    {
        return this.delegate.containsKey(key);
    }

    @Override
    public boolean containsValue(Object value)
    {
        return this.delegate.containsValue(value);
    }

    @Override
    public V get(Object key)
    {
        Object okey = null;
        if((okey = this.proxy.get(key)) != null)
            key = okey;
        return this.delegate.get(key);
    }

    @Override
    public V put(@Nullable K k, @Nullable V v)
    {
        return this.delegate.put(k, v);
    }

    @Override
    public V remove(Object key)
    {
        return this.delegate.remove(key);
    }

    @Override
    public V forcePut(@Nullable K k, @Nullable V v)
    {
        return this.delegate.forcePut(k, v);
    }

    @Override
    public void putAll(Map<? extends K, ? extends V> map)
    {
        this.delegate.clear();
    }

    @Override
    public void clear()
    {
        this.delegate.clear();
    }

    @Override
    public Set<K> keySet()
    {
        return this.delegate.keySet();
    }

    @Override
    public Set<V> values()
    {
        return this.delegate.values();
    }

    @Override
    public Set<Entry<K, V>> entrySet()
    {
        return this.delegate.entrySet();
    }

    @Override
    public ProxyHashBiMap<V, K> inverse()
    {
        return this.inverse;
    }

    public void injectSpecial(K key, K okey)
    {
        this.proxy.put(key, okey);
    }

    public void ejectSpecial(K key)
    {
        this.proxy.remove(key);
    }

    private class Inverse<V, K> extends ProxyHashBiMap<V, K>
    {
        private ProxyHashBiMap original;

        private Inverse(ProxyHashBiMap original, BiMap<V, K> forward, BiMap<K, V> backward)
        {
            super(forward, backward);
            this.original = original;
        }

        /**
         * Prevent creation of more Maps, just return the original
         * @return the original BiMap
         */
        @Override
        public ProxyHashBiMap<K, V> inverse()
        {
            return this.original;
        }
    }
}