The GIMP

Website: www.gimp.org/
Free for commercial use

The GIMP is a fully functional graphics manipulation package. There are versions for different operating systems, although the Linux version is most robust.

One of the most powerful features of the The GIMP is that you can write you own scripts and plug-ins, in a variety of languages. For example, you could write a script in perl to create a set of buttons (for a web page) from an array of strings. We've found the GIMP to be a very flexible tool.