Which popular apps prevent sleep on Mac OS X?

The main reason a Mac won't go to sleep is because of a 3rd party app. If there was a list of popular apps that have been known to cause sleep issues, it would make debugging this issue easier.

Which popular 3rd party apps are known to cause sleep issues?

Known to cause sleep issues:

• Better Touch Tool: Even when it is disabled, as long as it's running, sleep mode won't work.
• Stack Exchange Notifier: more info here.
• Symantec Encryption Desktop: more info here, Symantec's Official Reason here.
• NoSleep: App that does what it says. Although this is the main purpose of the app, for troubleshooting purposes it's good to check whether it's installed and accidentally on ;-)

Could potentially cause issues:

• Dropbox caused problems in Snow Leopard.
• Spotlight can prevent sleep, especially if you just installed Mac OS X.

Apps that have never caused sleep issues: