Mac OS X only: Free application SnowChecker runs a scan of your installed applications and tells you which may have problems in running in Snow Leopard.
(Click the image above for a closer look.)
If you've been hesitant to upgrade to Snow Leopard for fear of application incompatibility but you didn't feel like checking compatibility lists row by row to see how your must-have applications stack up, SnowChecker makes this process a breeze. Built on top of previously mentioned Snow Leopard Compatibility List, SnowChecker does all the hard work for you. Just download it, run it and watch for items with status alerts.
SnowChecker is a free download, Mac OS X only.