Certainly comparisions will be made to Google Desktop Search, which I'm already a fan of, but here's YATB (yet another toolbar) for IE. Rather than building it into the operating system or XP SP2, it's a viral MSN toolbar. Here it is, Microsoft's Desktop Search.
I've installed it, and I'm currently digging into how it works. Doesn't support Firefox, though. ;)
It has an X1-style type-ahead feature and rather than browser integration it's an Explorer Toolbar (I needed more things in my tray! Woohoo!)
Currently Indexing...if it indexes .CS files, I'm all in.
UPDATE: It DOES index source and XML! Yum.