I was all about CleanSweep and DiskDoubler way back when I was 15 and they promised to double my disk to a whopping 40MB.  Nowadays I usually shy away from these 3rd party clean up utilities.  Partly because they tend to be spammy and questionable themselves, and partly out of principal - shouldn't the OS handle keeping itself tidy?  But recently, with less than a GB left on my disk, and my system crawling, I decided to try out a few.  Here are some winners I found:

• JkDefrag - a free, faster, smarter defrag utility.  It still uses the windows defrag API, but can be scheduled, run as a screensaver, etc.  It also does some additional optimizations.
• SpaceMonger - a disk usage analyzer with a brilliant UI.  Not free, but worth it.
• CCleaner - a free clean-up-everything utility.  Temp folders, cache, registry, and all of that stuff the OS _should_ do, but doesn't (or at least doesn't do it all in one convenient UI).  Unlike similar apps, this one is adware free (if you uncheck the 'yahoo toolbar' on setup) and has a clean, simple UI.

Between those, I managed to free up 10+ GB!