Ram Shankar Yadav shared a handy tip the other day for finding documentation about .NET types on MSDN2.
 
You can just browse to http://msdn2.microsoft.com/library/ (fully qualified type name).
 
For example:
 
http://msdn2.microsoft.com/library/system.web.ui.webcontrols.textbox.aspx

I've already used this several times, and am considering writing a Visual Studio add-in to automate it. Thanks, Ram!