Kolourpaint is more or less an exact copy of MS Paint (at least it was when I left the MS sphere).
If you don't like KDE, try gpaint, gnome-paint or rgbpaint. They all use the same principles, but unfortunately are missing certain small features, which make kolourpaint my only KDE program.
Pinta might be worth a try too, but I haven't tried it so far. (Edit: Meh, it depends on mono)
Mtpaint is pretty mighty for its low weight, but unfortunately also kind of cumbersome. I don't actually like it, but still use it sometimes.