I managed to find a 333 mL water bottle that fits under my seat, which took some doing. Got a granola bar there too. That's just for emergencies though, since I'd rather stop and get a drink.
I've added some electrical tape and a bicycling wrench to my tool kit (the wrench is good for all the small bolts), and I need to add a tire patch kit under there as well.
Oh, I just added a thin pack of Wet Ones (always got 'em around for the Hordling's diapers).
My tail bag always has the small can of lube (the big one stays home) latex gloves, chewing gum, eyedrops, earplugs, pen and paper, swiss army knife, flashlight.
My pressure gauge is in my pocket.
Rain gear is weather-dependent, but it goes in the tail bag (on in the teeny Joe Rocket backpack).
And since I usually commute to work, the tail bag gets dress pants, shoes, and my lunch
:peeks at other lists: You're not allowed to wear earphones in Quebec, so I never have my ipod with me.
And the sunglasses are given. They're prescription and I can't see without 'em!