Simply the best you can get for free.

For a free application it has everything you need to optimize your machine. Simplified to the core functions, fast, and undisturbing. It is also getting better with every new release. Keep up with the updates and your mac will work all fine. Discrete meter in the menu bar shows the memory usage, the pop-up interface is very clean and slick. The disc cleaning function has lots of options. This app is also very sufficient to clean up absolete stuff after removing a no needed program. Simple, fast and free memory and disc optimizer.

neon-b about Dr. Cleaner: Disk, Mem Clean, v2.0.2