Easy way to disable sleep temporarily?

Hi,

I'm wondering if there is an easy way to keep the computer awake temporarily without having to edit the system preferences. My current strategy is to start iTunes playing with the computer set to mute. I need this functionality because the computer goes to sleep even if there are active processes running in my X11 windows. For example, I'd like to be able to flip a "no sleep" switch while I'm installing a big program via the terminal or ssh-ed into an external machine. I think this kind of temporary no sleep mode would be useful when I have to read documents on my computer screen so that the screen won't go dark every few seconds when I leave the mouse still. Once I'm done either reading or doing things in the terminal, I'd like to return my computer to my usual settings, which are set to engage sleep mode fairly quickly to conserve energy. Of course, I can always tap the mouse to keep the display awake when I'm reading and put iTunes on mute to keep the hard disk awake while installing, but I'm curious if there is a more elegant way to achieve the desired result.

Thanks,
Adrian