A bunch of
programs for
image manipulation. Available for
UNIX and
Windows.
It includes tools for just about everything one can imagine... It displays, converts, does effects, combines, and all kinds of other nifty things.
For heavier (and interactive) use GIMP is better, but you don't convert 1000 images from one format to other with GIMP, do you? =)
ImageMagick is webmaster's second best friend - is perfect for making thumbnails and with PerlMagic it makes mod_perl really fearsome.