StringWalker class:

public class StringWalker
{
	string _s;
	public int Index { get; set; }

	public StringWalker(string s)
	{
		_s = s;
		Index = -1;
	}

	public bool MoveNext()
	{
		if (Index == _s.Length - 1)
			return false;
		Index++;
		return true;
	}

	public char CharAtIndex()
	{
		return _s[Index];
	}
};

IndentWriter class:

public class IndentWriter
{
	StringBuilder _sb = new StringBuilder();
	int _indent;

	public void Indent()
	{
		_indent++;
	}

	public void UnIndent()
	{
		if (_indent > 0)
			_indent--;
	}

	public void WriteLine(string line)
	{
		_sb.AppendLine(CreateIndent() + line);
	}

	private string CreateIndent()
	{
		StringBuilder indentString = new StringBuilder();
		for(int i = 0; i< _indent; i++)
			indentString.Append("    ");
		return indentString.ToString();
	}

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

JSON formatter class:

public class JsonFormatter
{
	StringWalker _walker;
	IndentWriter _writer = new IndentWriter();
	StringBuilder _currentLine = new StringBuilder();
	bool _quoted;

	public JsonFormatter(string json)
	{
		_walker = new StringWalker(json);
		ResetLine();
	}

	public void ResetLine()
	{
		_currentLine.Length = 0;
	}

	public string Format()
	{
		while(MoveNextChar())
		{
			if (this._quoted == false && this.IsOpenBracket())
			{
				this.WriteCurrentLine();
				this.AddCharToLine();
				this.WriteCurrentLine();
				_writer.Indent();
			}
			else if (this._quoted == false && this.IsCloseBracket())
			{
				this.WriteCurrentLine();
				_writer.UnIndent();
				this.AddCharToLine();
			}
			else if (this._quoted == false && this.IsColon())
			{
				this.AddCharToLine();
				this.WriteCurrentLine();
			}
			else
			{
				AddCharToLine();
			}
		}
		this.WriteCurrentLine();
		return _writer.ToString();
	}

	private bool MoveNextChar()
	{
		bool success = _walker.MoveNext();
		if (this.IsApostrophe())
		{
			this._quoted = !_quoted;
		}
		return success;
	}

	public bool IsApostrophe()
	{
		return this._walker.CharAtIndex() == '"';
	}

	public bool IsOpenBracket()
	{
		return this._walker.CharAtIndex() == '{'
			|| this._walker.CharAtIndex() == '[';
	}

	public bool IsCloseBracket()
	{
		return this._walker.CharAtIndex() == '}'
			|| this._walker.CharAtIndex() == ']';
	}

	public bool IsColon()
	{
		return this._walker.CharAtIndex() == ',';
	}

	private void AddCharToLine()
	{
		this._currentLine.Append(_walker.CharAtIndex());
	}

	private void WriteCurrentLine()
	{
		string line = this._currentLine.ToString().Trim();
		if (line.Length > 0)
		{
			_writer.WriteLine(line);
		}
		this.ResetLine();
	}
};

Few samples:

Console.WriteLine(new JsonFormatter(
	@"{""parameter"" : ""value"" , { ""parameter2"" : ""value2"" },{ ""parameter3"" : ""value3"" } }").Format());
Console.WriteLine(new JsonFormatter(
	@"{""parameter"":[""value1"",""value2"",""value3""] }").Format());
Console.WriteLine(new JsonFormatter(
	@"{""parameter"": ""value with {brackets}"" }").Format());

…and results:

{
    "parameter" : "value" ,
    {
        "parameter2" : "value2"
    },
    {
        "parameter3" : "value3"
    }
}

{
    "parameter":
    [
        "value1",
        "value2",
        "value3"
    ]
}

{
    "parameter": "value with {brackets}"
}