At the command prompt outside of X, you could just highlight the entire screen and paste it into a text editor, if you are running gpm. 🙂
As for X, there are quite a few ways:
1. import -window root filename
The "import" utility comes with ImageMagick. Most distros install this by default, and all distros offer it as an option. You can give the filename a .jpg, .png, or .tif extension, and it will save it in the correct format automaticly.
2. ksnapshot
This is a program that comes in the kde-graphics package. It's easier to use than import, as it has a graphical interface, and lets you take screenshots of individual windows just by clicking on them. However, I think it can only save as PNG.
3. The GIMP
Open it up, then go to File > Acquire > Screen Shot.