I've been reading about pinvoke.net, by Adam Nathan quite a lot in blogs lately. As I am not doing any thing with interop at this time, I didn't feel compelled to go to the site. Well, I did now. My gosh, I'm impressed.
It's not just the content, either. The design and implementation of the site is wonderful, too. You can get to signature pages by searching, by browsing the directory, or by typing the name of the signature into a box at the top of the page.
To top it all off, Adam has created an add-in for Visual Studio that allows one to insert the correct signatures into code directly from the pinvoke.net site.
This is really good stuff!