NoMoreiTunes Safari Extension
NoMoreiTunes (via Safari Extensions) prevents Safari from auto-opening iTunes when you view a Web page for the iTunes store. You can still click the “View In iTunes” button manually. So far this is the only Safari 5 extension that I’m using.
I think Safari Extensions are the canary in the coal mine.
If I understand correctly, Apple has the ability to ban Safari Extensions from working by revoking certificates.
If Apple ever decides to do so for any reason other than sheer developer maliciousness, then we'll know that Apple's "Let's Be Evil" policy towards the iOS sphere will extend to the OS X sphere as well.
Until that canary dies, we're still living in a free world on OS X.