Memory Cleaner (http://memorycleaner.alicedev.com/) is now available at the Mac App Store. It costs US$5.99 and requires Mac OS X 10.6 or higher.
Memory Cleaner frees wasted memory in your computer to prevent your applications from slowing down. Every time you open a file on your hard drive, your computer uses up some memory. This is usually a good thing, but it can become full of stuff you don’t even need anymore. Memory Cleaner gets rid of this stuff.