Garett Rogers over at Googling Google discovered a few weeks ago that there is some dormant code in Google Calendar that appears to have only one possible use: completable events, otherwise known as tasks. For me, this is a killer, killer feature. I've already replaced Microsoft Outlook for email and calendar management with Gmail and Google Calendar respectively. Right now I'm using Backpack by the 43 Folders 37 Signals guys for my task list needs, and to be honest Google had better get it right when they do release the task list functionality, because 43 Folders 37 Signals has set the bar extremely high. The only thing that could get me to drop my beloved Backpack home base would be the ability to have a tightly integrated system with Gmail, Google Calendar and Google Task management (Gtask?).

Update - changed 43 Folders to 37 Signals after the mistake was pointed out in the comments. That's what I get for doing my writing at 2:00 AM!