Flash on a Mac
Seriously, how does anyone get any work done when this program has about a 33% chance of crashing every time you preview or publish a movie. This never happens on Vista or XP (well it does, but once a day rather than 3 times an hour).
On the flip side we set up a new Vista laptop yesterday. It got a virus by the time we had downloaded Avast and done the Windows updates. Typical.