I am not sure about open source projects. However Microsoft has added the Expression line of applications that support ASP.NET development. I think the ultimate goal is to allow better integration between development teams and ui design teams...but the Expression tools are pretty nice. (They also provide some competition to Adobe products on the graphics, silverlight, and html front...although nothing truely serious yet.)
MonoDevelop is an up-and-coming IDE. It's strictly not pure ASPNET but it's pretty good for something so new.
Are you limited to Windows or just ASP.NET in general? If the latter, check out MonoDevelop.
As of this writing, SharpDevelop 3.1 beta 1 was released only 13 days ago. Doesn't sound very dead to me. In fact it's the only choice on Windows if you want an IDE for the Boo programming language. And hey, it's even got a built-in profiler! I might have to check that out, thanks for reminding me to take another look.
Would VS.NET Express count? http://www.microsoft.com/Express/ . It's also VS.NET, but its simpler and free.
+1 good question Josh
I've tried the Mono stuff, and SharpDevelop, but I think the answer to your question might just be Visual Studio 2010.
I understand the desire to leave the Microsoft walled garden, I've felt it myself, and in that scenario, where you want to explore outside Microsoft, then of course there are many alternatives, but even those don't feel as complete as Visual Studio (I'm thinking Eclipse, XCode, the Adobe Flash environment to name a few interesting environments which are kinda fun to play with)
But in the Microsoft world, I really believe they're attitude towards development is very progressive, and fresh. Every Visual Studio iteration feels fresh, no more so than VS 2010, and I think currently other alternatives in the MS world are just playing catch up.
Embarcadero bought Borland's tools division a while back, and still publish Delphi and CodeGear RAD Studio.
Whilst it's not open source, it does have a budget behind it and a slightly different perspective - as well as a different selection of languages for .NET.