As are both Parallels and M$ Windows, with which M$ Paint comes included.
I'm pretty sure Photoshop will do anything you need for "pixel art" one way or the other, but admittedly, that's overkill...
Kat, if you are feeling adventureous, you could try to install an X Server for OS X (there's one on the OS X CD), install
XCode and finally download, compile, then install
XPaint, which is a X Window clone of M$ Paint.
EDIT: Even better, there's
Paintbrush for OS X! (For those who don't get the in-joke, "Paintbrush" was the name of M$ Paint before Win 95). I just tried it out and it's just as shit as M$ Paint in any way i can imagine.