I use system activity monitor. Works pretty well, plus it has a few bonuses like checking the battery percentage.
FYI: We aren't going to be seeing apps like this much longer. Apple has been contacting developers over the last week or two and making them remove this memory cleaning functionality from their apps. Sucks, a lot.
Apple is slowly removing the free memory features, yes. I asked a few developers about what they think their reason to do this is, and it's either to sell more 3GS devices with more memory, or that Apple doesn't approve of the apps doing something the framework isn't supposed to be doing. Anyway, iStat is the best regarding interface and information, but System information has amazingly accurate battery info.