This is going to sound like an advertisement, but CleanMyMac only really works the first time, when it cleaned out like 40GB. After that, it was only good for cleaning out caches. Now I use Squeeze. It, in the background, when your CPU's not being used for much of anything else, converts all your files to HFS+ compressed files. The files... don't look different at all. However, it only works on Snow Leopard 10.6.4. Oh wel.