Cachemate is good, but Titanium really works (root needed)
I paid for Titanium because my brand new Samsung Galaxy S 4G started having low phone storage on the first day. Couldn't move a bunch of stuff, so I just did Superoneclick and rooted it, then installed Titanium. I was able to free up space on the phone itself (it's loaded with bloatware) by moving some stuff, but it also has a cache cleaner that got rid of another 25 MB of data that I didn't need. So, short story is, if you just need to clean you cache, cachemate is probably fine, but if you need more robust options for system maintenance, consider Titanium.