Just drag and drop tracks into the BPM Analyzer's interface and let it automatically calculate the BPM and update the ID3 tags—meaning all the work you have to do is the dragging and dropping. You can drag whole folders (say, for example, your iTunes library folder) into the app, so it's a breeze to get those BPMs added without much effort.
The app did hiccup on me a couple of times with a few tracks, but for the most part it worked exactly as advertised. The MixMeister BPM Analyzer is a free download, works on Windows and Mac OS X. For an alternative BPM app and playlist builder for Windows, check out previously mentioned BeatScanner. Thanks Eric!
BPM Analyzer [MixMeister]