Image tweakers. Except for the fact that none will write GIFs (damn you, Unisys), these three free apps are all nice complements to Preview -- and would make good editors for use with iPhoto too. No pixel-level editing, but lots of useful whole-image transforms. ToyViewer is an updated NeXT app (nice retro icon) that does tone/color/size adjustments etc. and saves in many formats (with some hiccups); PixelNhance won't resize images, but offers a brilliant split-image before/after view; Goldberg offers a slew of image manipulations via QuickTime and can view PDFs and movies.