Road morph pump. Multi-tool. Spare tube AND patch kit. Tire levers. Cable lock. This is all I commute with. Lockers (both bike and personal), Showers, clothing, and food are provided at work.

This can fit inside some of the larger seat wedge bags no problem, like a RavX Maxi. The cable lock is just in case you need to lock the bike up when you make a stop somewhere, or in case you break down and locking the bike up and comeing back for it later is a better option than pushing it. Leave the heavy duty locks at work so you don't have to lug them back and forth.

Seat bags and panniers are not secure, so if you lock the bike up outside, take them with you. Or decide that you are risking about $75 worth of stuff for the convenience of leaving it on the bike. I did this for two years. Two years of convenience, never had anything taken in my low crime lockup area. For the ultimate convenience, get a topeak MTX rear rack and an MTX bag. You just slide the thing on and off in seconds, plus you will have more capacity than a seat wedge.