For roadside repair purposes --

Minimum: a basic multi-tool, pump, tube / patch kit, good tire levers, wrench to remove wheel (as necessary), zip ties.
More hardcore: spoke wrench, chain breaker.
Insane: first aid kit, extra brake cable, spare spokes, spare tire, spare pedals, hubs, wheel truing stand, etc.

The minimum repair kit will prevent you from having to walk if something stupid happens -- like your handlebars loosen up or your seat post suddenly drops or whatever. And it's not just that you haven't kept up with bicycle maintenance: you never know what idiot with an allen wrench will fudge with your ride when you're not looking.

*Good* tire levers. My tire / rim combo is pretty tight, so levers are essential. Cheap levers will snap and all of the force that you were exerting on the lever will suddenly be exerted on your knuckles, courtesy of spoke nipples. Does that sound like experience talking? Yep. Let's just say I should have gotten the $2 levers.

Now go read everything you can in the Commuting forum. Figure out the rest as you go.