<p>I've kept up to date with new features in the C# language as it's moved from version 1 through version 3.  I haven't done such a good job keeping up to date with ASP.NET.  I feel like some of the post version 1 features are not so good (e.g. the AJAX framework) or are just not that useful to me (e.g. the membership framework).  Can anyone recommend any new killer ASP.NET features that might have gone unnoticed?</p>

I'm still learning ASP.net so I can't tell you exactly, but if you look through
http://www.asp.net/learn/
you'll probably find a few new gems, there's even a 3.5 section.