This question comes up a LOT. Unlike Windows, most Macs will run just fine without much user maintenance, but there are many recommendations folks run. You will probably hear a lot of folks recommend running "Repair Permissions", which is a topic of much discussion. Some will also tell you to get a copy of Cocktail or Onyx.

Personally, I spend as little time as possible maintaining my computer. I sometimes leave it on overnight to allow the built-in scheduled maintenance routines to run (they are divided into daily, weekly, and monthly scripts and do not run when your machine is asleep). Periodically I run DiskWarrior, but it's infrequent.

The real answer to the question is that there really isn't anything you HAVE to do. Lots of folks will give you their recommendations (some of which I consider voodoo), but you'll be just fine most of the time if you simply use the machine and don't worry much about it.

More important is the practice of backing up your data, but that's another discussion.