inkscape
Also uses X11 (like the GIMP does) for its mac port. As do many of the viable alternatives really. What's wrong with your X11? It might come down to a choice of fixing it, using something inadequate, or paying for pixelmator/PS elements.
+1 for Paint.net. There is also a simillar cross platform program in development (by another dev) called pinta and available for mac. Its awesome, but also uses X11 for some things... worth a try.
One nice free image manipulator that does run natively is seashore. Its not that full featured, but does what it does very well and certainly is worth a download.