Gimp = Image manipulation.
Inkscape/Sodipodi = vector image creation.
Scibus = Desktop layout and publishing.
Cinerella = hardcore professional video editing (not fun to use), Kino = normal video editing.
Blender = free 3d editing software, Maya = commercial 3d software
Some people like to compare Gimp vs Photoshop and stuff like that. whatever, each program has it's purposes and photoshop is one the programs held in highest esteem by a lot of people. Of course it's not nearly as great as people like to assume it is, most of them simply don't know what they are talking about. Gimp is a nice program and allows you to do everything that you can do in Photoshop, although the user interface isn't as nice.
Once you get used to it, Gimp is a pleasurable program to use.
A variation of Gimp is Cinepaint (formally known as Film Gimp) and it was created by people who needed a editor for film. It mostly has to do with what is knows as color depth. Printed paper and photos generally don't need more then 8bit per pixel color depth (24bit color, or 32bit "color" when you add 8 bits for a alpha channels which is what Windows "32bit" color depth means). This isn't because standards are low, but it's because they deal with reflected light (and the CRT tubes aren't that great and LCD is worse) and no matter what technology you throw at them you are never going to get past those limitations. However film (movies, not photograph.) has much more color/light depth so Film Gimp supports full 32bit color.
edit:
Actually I am mistaken. Cinepaint will support 32bits PER channel.
Up to 128bit color (RGBA)
It's used by studios like Rhythm & Hues, Sony Pictures Imageworks, DreamWorks, ILM. Used in movies:
like 2 fast 2 furious, Stuirt little and others.
Hell if you go into the extended DVD version of Lord of the Rings: Two Towers, they show images of a developer working on a KDE desktop.
pic
IIRC they used Cinepaint to draw in the arrows flying around in a few scenes in the Lord of the Rings stuff (at least in the later movies)
Of course most those studios use Linux quite a bit nowadays, having displaced much of the old school Unix stuff they used in the past. Lots of custom apps, tools, programmers and stuff like that. So it's not a big supprise they use Linux for much stuff (and windows of course). Lot of it is clusters compiling 3-d stuff. Most of the CGI stuff in the Lord of the Rings (and other movies) were rendered on Linux clusters. Which of course, you and I don't get to play around on any of that stuff.
Of course for most consumer/prosumer stuff it's going to be Macs first, Windows second, and Linux dead last in terms of software.
Inkscape/Sodipodi aren't as mature as Adobe app's are and it's shows. But again they will get done what is need to be done with little fuss.
And many others besides that.
http://www.linuxartist.com/
For sound and music editing and creation there is a bit more to choose from. You have all sorts of stuff, like Ardour is a good example of a high quality linux audio app.
http://ardour.org/
http://sound.condorow.net/
http://www.linuxdj.com/audio/lad/index.php3
Among others.
IMHO Linux and free open source software art stuff isn't up to the big commercial stuff's quality and capabilities that you can get if you have enough money, but it's hell of a lot better then most free stuff you get thru normal Windows channels.
Unless you warez your stuff, then it's all free.
Spoken like a true nerf.