For those deciding at the last minute to bike to work on Bike To
Work Day, here's a handy dandy checklist of things to have and do. I
recommend getting this stuff together the night before you ride.

Check weather forecast.

The bike:

Tires inflated.
Saddle adjusted.
Brakes adjusted and operating.
Gears adjusted.
Chain lubricated.
Lights work; batteries charged.
Flat repair:
Spare tube and/or patch kit.
Tire levers.
Pump or CO2.

Clothing:

Under five miles or so, nothing special is really needed.
For longer distances, bike shorts are nice to have.
Rain gear if necessary.
Safety gear: Gloves, eye protection, helmet, knee pads, elbow pads,
etc.
Backpack or some other way to carry "stuff." Contents may include:
Bike lock if necessary.
Cash for emergency public transportation fare.
Clothing & shoes for the office.

Personal hygiene items:

Towel & washcloth.
Soap.
Shampoo.
Hair care items.
Deodorant.
Lunch.
Wallet.
Office badge and keys.
House keys.
Cell phone, camera, whatever other gadgets you like to carry.

This is a fairly minimal list. You can go crazy and do much more,
but this is generally about what most bike commuters should need.