try Graphic Convertor, which is already in your applications folder. more of an image editor, it is still capable of image creation. it's more powerful than paint, and apple ahs already paid for it for you so it's free.
MacPaint was bundled with early Macs in the 80s, right? ... Nah, that's not what you're talking about. I think Graphics Converter does what you're looking for, but of course it's not free - unless it's bundled with your computer (as has been said).
If it's _not_ bundled with your computer, you might find free (or almost free) applications that do similar things on versiontracker.com or macupdate.com ...
I just had a quick search and came across the free paint app called ArtRage (download here). It's pretty good - take a look at the gallery of images created using ArtRage. wow!
It's one of the best-looking paint apps I've ever seen. very impressive. However, its features are somewhat limited. Depends what you're wanting to use it for I guess.
There's always GIMP, bit it's probably a lot more complex than what you're after (closer to Photoshop that Paint). Can be difficult to install also.
It's a nice little app, ArtRage, but certainly not a replacement for MS Paint or Graphic Converter... GIMP might be too complicated... I'd go with Graphic Converter. For a while, you can use it for free without too much hassle.