If you are working with the Ajax Control Toolkit, sometimes the assembly will become corrupted in your Temporary ASP.NET Files folder. Here are a set of steps that might help you get back up and running. This is only relevant to developing with the built in web server, Cassini....
The long awaited release of the .NET Framework Library source code for debugging purposes has arrived. Scott Guthrie announced it here . Scott Hanselman and Peter Bromberg blogged about it here and here . One of my mentors, John Robbins from Wintellect...