Been thinking about this and I've never seen a thread about the lowly tool bag.
Phillips and flat blade screwdriver (thought about combining them into a 6 way screwdriver but there are some things you just need a long flat blade for)
Small crescent wrench.
10mm combination wrench (nice not having to hunt one down to do a pre-ride torque check)
A couple of master and half links
Assorted zip ties
Gallon size Ziploc bag (gloves and towels are in this and it's a nice thing to put a greasy chain in if you ever have to remove it to pedal home)
About 5 sheets of paper towel rolled up
Latex gloves (nothing's worse then greasy hands after putting a chain back on)
Extra bungee cords
LED tail light (my tail light quits before the headlight does so it's a little insurance in case of a low battery)
I plan on adding a small bottle of 2 cycle oil, just enough for a fill up of my 32oz. reserve tank
:edit: I may just toss my GPS in there too. Today was overcast and I got a little lost taking an unfamiliar road. Ended up turning into a dirt road with all sorts of twists and turns and I ended up turning the wrong way and heading north when I thought I was going south. By the time I figured out my mistake I was a LONG ways away from where I thought I was. Damn glad I have that extra gas onboard now.