Needs a separate mention just for the $29 a year all music you can convert to legal feature. This is the "jaibreak" we've all been waiting for!
1
Airdrop
AirDrop is a new way to share documents between Macs using a peer to peer WiFI network. This can revolutionize file (and p0rn) sharing on wifi campuses
4
OS X Lion for $30
Only $30 and can be installed on as many Macs at home. Borrows heavily form the iOS interface. Mobile is changing desktop.