Depends where I am but the seat bag has a tube, multi wrench, cell, wallet and keys (couple bandaids in the wallet), and maybe a Clif bar. On the seat tube I've got a Topeak repair kit. I've also got a pump mounted and at least one water bottle.

If I'm going for a ride in the hills or someplace more remote, I strap on a Mountainsmith lumbar pack or strap it to the rack and it has a number of survival related items including shelter and first aid. I do SAR so if I'm not prepared and get stuck, I'll hear about it forever.