Screenshot Grabbing

Using imagemagick

imagemagick is a collection of tools and libraries for image manipulation. Most distribustions come with imagemagick .
To take a snapshot using Imagemagic:

import -window root screenshot.png

import manual page

Using xv

xv is another image manipulation program that can deal with almost all file formats. xv is not included in distributions installations, but packages are available for most. Example usage:

xv -grabdelay 2 myimage.jpg
xv manual page

using gimp

To grab an image of the screen or a program in Gimp:

File -> Acquire -> Screen shot

using Framebuffer Console

Use fbgrab to grab a screenshot in a framebuffer console.

fbgrab filename.png

using kde

KDE comes with a handy program called ksnapshot that lets you with ease grab a screenshot of your desktop or a single window.

scrot

scrot is a small (66 kB) screen shot grabber based on imlib2. It has lots of options for autogenerating filenames, and can do fun stuff like taking screenshots of multiple displays and glueing them together. thanks to miztic for mentioning it

using xwd

xwd (manual page) is a part of XFree, so chances are high you already have it installed. It can dump screenshots to .xwd files. This is not a common format. After taking a screenshot using

xwd -root -out test.xwd

the only programs I found capable of opening it were Gimp and xwud (manual page). thanks to lude for mentioning it