If you've ever used Subversion (SVN) on Windows, chances are you've used TortiseSVN. (If you haven't, definitely check it out.) It works as an extension to Windows Explorer. What this means, is that you don't have a separate UI for SVN - you do everything in Explorer. I was really struck by how cool this was when I was working on an old project using an old version of Perforce; it seemed barbaric to have to keep switching between the SCM UI and Explorer.
Until recently, there wasn't anything like that for the Mac, at least not anything that was free/open source and that I knew about. Then, along comes SCPlugin. From the web page: "The goal of the SCPlugin project is to integrate Subversion into the Mac OS X Finder. The inspiration for this project came from the TortoiseSVN project."
As of this writing, it's at version 0.7, which means it has a few rough spots, but for the light use I've put it to so far, it's been mostly pretty good. And since it's 0.7, it has nowhere to go but up.