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Depends...

...on the program. In sixtyforce (www.sixtyforce.com), for example, you turn on the show framerate check box in the prefs. In Quake III it's something else. I don't play QIII so I dono't know.
 
Quake

Hi , here is the command for Quake 3:

cg_drawfps 1

+ To turn it off again, type: cg_drawfps 0

Hope it helps
 
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