Keeping the mac awake when running certain applications? I need to keep my MacBook Air awake when running certain applications. But when I put the display to sleep, often the whole system suspends after a few minutes.

Specifically I run Air Video Server on the Mac to stream/decode videos to my iPad. But the Mac keeps sleeping and thus stopping streaming videos. 

I tried to use Caffeine to keep the Mac awake, but it looks like it has lost its magic in Mountain Lion – it couldn't keep the Mac awake to continue running services after I put the display to sleep.

Wimoweh (another sleep-preventing app) couldn't detect whether Air Video server is running, so it's probably won't be useful in this situation.

So is there any other app that can keep a Mac running services with the display asleep? I'm not keen to install OS X Server on this laptop since it's more of a once-in-a-while thing and most of the time I need the Air to really sleep if it's unused.
I use caffeine to avoid the system to sleep, you can configure the time is "awake" as well: http://lightheadsw.com/caffeine/

Edit to update as caffeine hasn't been updated for a long time, Amphetamine seems to work OK https://itunes.apple.com/us/app/amphetamine/id937984704?mt=12