Keep malware off your Mac
In the last year, the Mac has seen its first serious malware attacks, using third-party conduits such as Java. Apple loves people to think that malware is a Windows problem, but it's been forced to add antimalware capabilities to OS X Lion and enhance them in OS X Mountain Lion with the new Gatekeeper facility.
By default, only apps purchased from the Mac App Store install in OS X Mountain Lion. But you can let third-party apps with a valid Apple developer ID install, no matter where obtained. And you can disable Gatekeeper -- including for just an individual installation that you trust via a contextual menu option.