Also, make sure you check out Philip’s website, where he has a ton of other great open source code for OS X.
What I like best about SPGroupedTabView is that it’s a fresh take on the, frankly, tired source list concept. Clearly, this won’t scale for all scenarios, but I can imagine that this could work well for apps in the vein of, say, Twitter or Facebook clients that are trying to differentiate themselves from the now-de facto standard of vertical tab bars.
What do you think? Let me know if you use this control in your apps, if you think I’m off base about novel tab UIs, or anything else.