Siri is obviously the breakout feature in Apple's latest handset. Love it or hate it, you can't deny that it's a crowd pleaser (when the servers are up). Even the critics that say that the system is too limited right now, but we all have to agree that Siri has some great potential.
The potential lies with third party developers. Currently Siri can't launch apps or perform tasks inside third party applications, but it's coming. And the folks behind the popular task manager Remember The Milk are leading the way...