i commute by bike (when i have a job ). actually, i do everything by bike since my car broke down and i had to sell it. i ride an old peugeot conversion that's been surprisingly sturdy. i have a large mess bag (the kemmer special) for carrying my stuff. for work i would take an extra shirt (in case i got sweaty), a hoodie and gloves in case it got cold while i was inside, my lock, other personal stuff, and my tool bag.

the tool bag is one of those zippered bank bags, and i keep tire tools, an extra tube, hand pump, multitool, 15mm wrench, small adjustable wrench, knife, $20 for a cab ride in an emergency (well, i spent the $20, but i left myself an IOU) and a spare red blinky light.

i keep a red blinky light attached to my seat post, but i keep the spare to clip on my bag if i'm riding in the dark and it's raining or foggy. i also keep a little yellow rain jacket in one of the interior pockets, folded up all nice and neat, as well as a couple of extra rags and a bandanna to cover my face/neck when it's really f'ing cold.

obviously a bunch of this stuff i can take out to save space when i go grocery shopping, but for commuting or daily riding, i don't want to be caught without something when i need it. and it doesn't really weight all that much, and the bag has stayed comfortable no matter how it's loaded most of the time.