Mac OS cleans up pretty well all by itself - it runs its own maintenance programs in the middle of the night (if it's on); if it's not, they will be run once it wakes up. Except for repairing permission possibly after an OS or other software install/update, there isn't really much to do. I use Disk Warrior occasionally - it repairs the disk directory; but that is only necessary occasionally.
Adding to what babowa posted, see these
Mac Maintenance Quick Assist,
Mac OS X speed FAQ,
Speeding up Macs,
Macintosh OS X Routine Maintenance,
Essential Mac Maintenance: Get set up,
Essential Mac Maintenance: Rev up your routines,
Maintaining OS X,
Five Mac maintenance myths, and
Myths of required versus not required maintenance for Mac OS X for information.