In case you didn't piece it together from the name, RegDefrag will defragment your registry. In layman's terms: the program works to optimize your registry by removing gaps, fragments and wasted space in Windows registry files.
RegDefrag begins by analyzing your system's registry. If your registry is sufficiently fragmented, RegDefrag will give you the option to defragment. Unfortunately, there are no backup options (unless RegDefrag performs them behind the scenes, but we wouldn't count on it), which means you're out of luck should RegDefrag decide to remove or defragment the wrong files.
In our tests, RegDefrag recovered over 3 MB of space, which was about 10% of our computer's registry. Not too shabby.
RegDefrag is free, and compatible with Windows Vista, XP, and the ghosts of Windows past.