Useful graphics software

Here is a list of some graphics software I find useful:

Raster graphics

The GIMP A cross-platform photo and image editor
Phatch A cross-platform photo batch processor
IrfanView An excellent image viewer and batch processor for Windows (which runs fine under Wine as well)

Vector graphics

Inkscape A cross-platform vector graphics editor, which is easy to use
sK1 Another cross-platform vector graphics editor, aimed more at professional graphics designers

Image conversion

There are plenty of graphical tools available for batch image conversion (e.g. Irfanview for Windows/Wine, Phatch for all platforms).
But with some help of ImageMagick's convert command it's also very easy to batch-convert pictures on the commandline. For instance, to convert a set of BMP images to JPEG, you can do: for i in *.bmp ; do convert "$i" "${i%bmp}jpg" ; done

This probably requires you have bash as a command-shell. For those unfamiliar with shell-scripting: The above line is basically a one-line program that loops over all files ending in .bmp, stores the filename in a variable called i, and passes it on to convert. The %bmp part strips off the bmp extension, after which jpg is added to the output filename. The quotes are there to make sure that it still works when there are spaces in the filenames...