Mac's don't need defragging as OS X automatically writes 2Gb or below sized files to one contiguous space on the drive. There are certain cases where's it's needed to free up a large spaces on the drive, but not something everyone has to do.
CCleaner ® for Mac. A Mac collects junk and unused files just like a PC. Find and remove these files with the click of a button so your Mac can run faster. Speed up boot times with easy management of Startup items. Clean your Mac of old clutter. Clear up your recycle bin, temporary files and folders, broken permissions and more.
Far as other Mac maintainence, there really isn't any except to backup one's files regularly. Keeping Software Updated and run a web browser check to make sure the plug-ins are current.
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For Safari and Flash, this check is more accurate
Most of those cleaning programs are junk, especially MacKeeper.
If your friend got onto your computer and visited a bunch of adult sites, then I would recommend something like OnyX, doing ALL the cleaning and maintenance aspects and rebooting, perhaps followed by a Disk Utility Erase free space.
CCleaner beta is out for OS X, used on PC's and combines cleaning and scrubbing of the drive if setup correctly.
Mac Drive Cleanup
If your interested in performance, read my post here