If you find the GIMP a little too complex (it is more of a Photoshop clone than a MS Paint clone) there are a few alternatives:
Seahorse - closest to MS Paint in style, this is a stripped down version of the GIMP. This is a native OS X app.
Inkscape - similar to MS Paint (but more powerful) but runs in the X11 environment.
ArtRage2free - this is truly a painting/drawing app and is very nifty. Free version doesn't allow you to use all the tools available or layers, but is very impressive nonetheless.
NeoOffice or OpenOffice.org have a drawing component as part of their suite of office tools. Both are free - NeoOffice runs natively in OS X whereas OpenOffice.org runs in the X11 environment.
Not free but another similar app is
iDraw