One of the best find that I had came across while looking into the various Ajax frameworks is Ajax.NET. The Ajax.NET AjaxPro library is the first .NET implementation that allowed for Ajax functionality within the .NET framework. It is extremely easy to use and implement and is a better way to implement Ajax functionality than using the bloated UpdatePanels in ASP .NET 2.0.

For example to serialize C# objects to JSON strings, you just need to go:

public String convertToString(Object pojo)
{
        return AjaxPro.JavaScriptSerializer.Serialize(pojo);
}

To deserialize objects from JSON strings, all you need to do is:

public object convertToObject(String str, Type className)
{
        return AjaxPro.JavaScriptDeserializer.DeserializeFromJson(str, className);
}

And best of all, it is open source, so the source code for the library is available for download. Although Ajax functionality is better supported now under the 3.5 framework, I still mainly use Ajax.NET because of its simplicity and also a lot of my old code still uses AjaxPro.