A Simple c# class for creating well formed JSON strings when serializing classes with Json.NET is overkill.

JsonBuilder.cs

public class JsonBuilder
	{
		private readonly StringBuilder _sb;
		private readonly Stack<bool> _hasPreviousProperties;

		private bool RequiresComma { get { return _hasPreviousProperties.Count > 0 && _hasPreviousProperties.Peek(); } }


		public JsonBuilder() {
			_sb = new StringBuilder();
			_hasPreviousProperties = new Stack<bool>();
		}


		public JsonBuilder WriteObject(string name, Action<JsonBuilder> buildChildren)
		{
			WriteCommaIf(RequiresComma);

			if (_hasPreviousProperties.Count > 0) {
				_hasPreviousProperties.Pop();
			}
			_hasPreviousProperties.Push(false);

			_sb.AppendFormat("{0}:{{", name);
			buildChildren(this);
			_sb.Append("}");

			if (_hasPreviousProperties.Count > 0) {
				_hasPreviousProperties.Pop();
			}
			_hasPreviousProperties.Push(true);

			return this;
		}


		public JsonBuilder WriteObject( Action<JsonBuilder> buildChildren )
		{
			// if object is anonymous don't write comma, the coder should have done it
			_sb.Append("{");

			if (_hasPreviousProperties.Count > 0) {
				_hasPreviousProperties.Pop();
			}
			_hasPreviousProperties.Push(false);

			buildChildren(this);
			_sb.Append("}");
			
			return this;
		}


		public JsonBuilder WritePropertyIf(string name, object value, bool onlyIf) {
			if (onlyIf)
				return WriteProperty(name, value);

			return this;
		}


		public JsonBuilder WriteProperty(string name, object value)
		{
			WriteCommaIf(RequiresComma);

			_sb.AppendFormat("{0}:", name);

			WriteObject(value);

			if (_hasPreviousProperties.Count > 0)
				_hasPreviousProperties.Pop();

			_hasPreviousProperties.Push(true);

			return this;
		}


		public void WriteNull()
		{
			_sb.Append("null");
		}


		public void WriteArray(IEnumerable values)
		{
			_sb.Append("[");
			_hasPreviousProperties.Push(false);

			foreach (object value in values)
			{
				WriteCommaIf(RequiresComma);
				WriteObject(value);

				_hasPreviousProperties.Pop();
				_hasPreviousProperties.Push(true);
			}

			_sb.Append("]");
		}


		public StringBuilder Raw { get { return _sb; } }

		public override string ToString()
		{
			return _sb.ToString();
		}

		private void WriteCommaIf(bool ifOnly)
		{
			if (ifOnly)
				WriteComma();
		}

		public void WriteComma() {
			_sb.Append(",");
		}

		private void WriteObject(object value)
		{
			if (value == null)
				WriteNull();
			else if (value is string)
				WriteString((string)value);
			else if (value is bool)
				WriteBoolean((bool)value);
			else if (value is int)
				WriteInt32((int)value);
			else if (value is double)
				WriteDouble((double)value);
			else if (value is IEnumerable)
				WriteArray((IEnumerable)value);
			else
				_sb.Append(value.ToString()); // ewww
		}

		private void WriteBoolean(bool value) {
			_sb.Append(value.ToString().ToLower());
		}

		private void WriteString(string value)
    		{
        		_sb.AppendFormat("\"{0}\"", value.Replace(@"\", "")
                                .Replace("\"", "\\\"")
                                .Replace("\r","\\r")
                                .Replace("\n","\\n")
                                .Replace("\t","\\t")
                                .Replace("/","\\/")
                                .Replace("\b","\\b")
                                .Replace("\f","\\f"));
    		}

		private void WriteInt32(int value) {
			_sb.Append(value);
		}

		private void WriteDouble(double value) {
			_sb.Append(value);
		}

	}

you're not escaping any of the characters that require escaping; such as \r
see http://json.org/ (white box on the right lists characters that should be escaped)

    private void WriteString(string value)
    {
        _sb.AppendFormat("\"{0}\"", value.Replace(@"\", "")
                                         .Replace("\"", "\\\"")
                                         .Replace("\r","\\r")
                                         .Replace("\n","\\n")
                                         .Replace("\t","\\t")
                                         .Replace("/","\\/")
                                         .Replace("\b","\\b")
                                         .Replace("\f","\\f"));
    }

Only thing missing here is unicode character encoding (UTF-16?)

Thanks @jitcoder, will update

Where is BuildChildren method