Windows: the
print key on your keyboard will take a screenshot and store it into the clipboard. Your favourite image processing tool will allow you to save it in different formats.
Linux/KDE: there is a tool called KSnapshot, in the Graphics menu, which will save your screenshots in .png format in a predefined subdirectory of your home
Linux/Gnome1.4 : there a screenshot tool in Applets -> Utilities, which behaves almost as KSnapshot.
Warning its default image format is poor quality jpg, but you can easily configure a png output.
Linux/Gnome 2 : a keybinding should be enabled by default to take screenshots.
Mac OS X : ask the DJ, he should know
IMPORTANT before taking your shot, please deactivate the Overlay function among the video output preferences. If you don't, you will get a screenshot of the overlay colour, which is probably not the output you are expecting.