In the pockets: For a solo ride: Cell phone & keys. For a group ride add a map/cue sheet, some real ID and a few bucks.
In the seatbag: Spare tube, tire levers, multitool, $5, glueless patches, photo copy of ID and Ins. Card and my "latte" lock (ie. a cable lock just strong enough to secure the bike for 5 minutes outside a coffee shop).
For commuting: hand grenades.