Wimoweh is a menu bar application that allows you to have better control over how your Mac sleeps
Shows which Apps are preventing your Mac from sleeping and why.
Allows you to always stay awake when certain applications are running and automatically sleep when they've finished. This is great for apps that Lion seems to 'ignore' even though they're working or if you're waiting for a lengthy job to finish and want the Mac to sleep afterwards.
Set up a countdown to keep your Mac awake for a certain amount of time
And of course, you can just keep your Mac awake indefinitely.
Mac OS X 10.7
Mac OS X 10.8