I recently worked on an iPhone app, and had to do a little bit of graphic design.
I didn't have the money for the adobe suite and needed free/open source versions of photoshop and illustrator, for bitmap and vector graphics respectively.
This two programs are well worth the download:
bitmap : http://www.gimp.org/
The new single window mode makes this app far more usable.
vector : http://inkscape.org/
