I do strongly recommend InkScape, I've never seen it discussed on these forums, but it is an invaluable tool for creating nice, vector art. That said, it can also export to raster once you've created your art. I've found that it makes it extremely easy to make typical app style art, with lots of tutorials on it around the web.
Inkscape is awesome tool! I always prefer it over Illustrator, mostly because of Illustrator's cumbersome vs. Inkscape's simple but powerful UI.