Download Yahoo! Widget Engine
Download XBOX Friends Widget
clicking on a name
It just sits there on your desktop (below everything else).
Another nice feature is that if you click on one of the games in somebody's gamertag it will bring up a page comparing your accomplisments in that game side-by-side with theirs. I have it set to hide my offline friends, but that can be expanded. Love those little details about what you're playing (I like seeing everybody's Oblivion progress).
A very nice little tool I've been using to spy on my friends. Thought you all might enjoy it.