MacOS X: Onyx

Titanium Software has a free optimization tool called Onyx.
As per Mark’s word:

Titanium’s OnyX is a nifty little OS X maintenance program – runs the cron scripts, repairs disk permissions, clears caches, etc.

Useful? probably. If someone wants to buy me a Mac, I’ll be happy to test it for you. Otherwise, I’ll take Mark’s word for it.