There are a lot of handy Visual Studio 2005 add-ins out there that, for a variety of perfectly legitimate reasons, have yet to be updated for Visual Studio 2008.
Before you go looking for replacement add-ins, though, you might want to check out a recent blog post by Mohamed Ahmed Meligy called Tip: Try this to make a VS 2005 add-in work with VS 2008. As the title implies, the author discovered a quick process for making sure that his Visual Studio 2005 add-ins would work on the newer version of the IDE.
The caveat here, of which Meligy is quite aware, is that this process may not work with all Visual Studio 2005 add-ins. Nonetheless, it certainly can’t hurt to try it out, especially if you have come to rely on a particular add-in so much that you think it’s part of the IDE itself.