A number of extension / utility functions that come in handy for almost every project that I've worked on.

Util.cs

public static class Util
{
    public static Vector3 Horizontal(this Vector3 v)
    {
        return Vector3.ProjectOnPlane(v, Vector3.up);
    }

    public static bool InBetween(this float val, float a, float b)
    {
        return (val < b && val > a) || (val < a && val > b);
    }

    public static T ToEnum<T>(this string value)
    {
        if (string.IsNullOrEmpty(value))
        {
            return default(T);
        }

        return (T)Enum.Parse(typeof(T), value, true);
    }

    public static FieldInfo[] GetFieldsMarkedWith<T>(object obj) where T : Attribute
    {
        return obj.GetType().GetFields(BindingFlags.NonPublic | BindingFlags.Instance)
                .Where(x => x.GetCustomAttributes(typeof(T), true).Any()).ToArray();
    }

    public static Type[] GetChildrenTypesOf<T>()
    {
        return (from domainAssembly in AppDomain.CurrentDomain.GetAssemblies()
                        from assemblyType in domainAssembly.GetTypes()
                        where typeof(T).IsAssignableFrom(assemblyType) && assemblyType != typeof(T)
                        select assemblyType).ToArray();
    }

    public static void SetAlpha(this Material m, float a)
    {
        var c = m.color;
        c.a = a;
        m.color = c;
    }

    public static void SetAlpha(this Image m, float a)
    {
        var c = m.color;
        c.a = a;
        m.color = c;
    }

    public static T Random<T>(this IList<T> list)
    {
        return list[UnityEngine.Random.Range(0, list.Count)];
    }

    public static void WaitForSeconds(float seconds, Action function)
    {
        Observable.Timer(TimeSpan.FromSeconds(seconds)).Subscribe(x => function());
    }
}