I've given a cursory look at some of the other lists and they look pretty good but FWIW here's mine for a 21 mile round trip in the Boston area.

Bike:
Solid and simple. (mine's a 9 speed REI Buzz all black matte finish).
Brooks B15 saddle
Racks front and rear.
2 bungee cords
Milk crate back.
2 front lights- handlebar mounted.
1 helmet light
rear light mounted on rack
1 blinkie attachable to clothing/helmet/bike
bell
Performance "campus" pedals (clipless one side, platform other)

"Stuff"
Messenger bag (throw it in milk crate or sling on shoulder as needed)
carries rain gear
extra clothes
extra batteries for lights
cell-phone
and
Tools:
mini-pump
spare tube
patch kit
bike multi-tool
tire levers (METAL- plastic breaks in winter)
small "leatherman"- I like having the pliers


Clothes:
Helmet
Showers Pass rain jacket- I carry it only when rain threatens
Rain pants " " " "
Shoe covers- only use in cold, wet, weather
Keen cycling shoes
Gore Wind Stopper Vest by Mountain Hard wear- use it or carry it almost all the time (except mid-summer)
Gloves- cycling gloves- often but not everyday, winter thin layer gloves, then thicker gloves then Gore-tex mitten shell for cold wet winter.
Layers, layers, layers.