I've been playing with the Google Deskbar a bit this week. It's not quite the no-brainer-install that the Google Toolbar was. Since I almost always have a browser open, I always have the Google searchbox in the Google Toolbar close by. And I hate losing the horizontal space in the Windows Taskbar.
Still, I like two aspects of it. The first is that the results display in a compact, retractable window instead of opening a new browser window. Secondly, because it accesses Google Calculator, I use it as such. It's a lot faster than firing up Excel or Windows crappy calculator, and it allows me to enter expressions.