Sun glasses because we are blind mole people in those lovely days of rain.

A ton of change because half the meters don't like the cards they say they are gonna accept but stick their toungs out at you when you're late

A spare hoodie for those days that start out so but just don't stay that way ( even in the summer, most eves get chilly enough to warrant it)

Can't think of a 4th that hasn't already be discussed

A pair of shoes you can actually walk in! I think this applies mostly to women--I've never known many men who would put up with the pain and limited mobility that most women get from their shoes on a daily basis. But whether it's breakdown or a road-shattering earthquake, there may come a time when you'll care less about looking fashionable and more about just getting somewhere.

* Hi-tech insulated travel mug from REI
* Hands-free phone device
* Gore-tex rain gear (breathable waders optional for north suburbs)
* Anti-mildew wipes
* Starbucks gift card

For me, the list is a little different:

* jumper cables (because I leave my lights on all the time)
* 75 grocery bags
* mound of newspapers, books, computer parts and food wrappers filling back seat
* dozens of uncased CDs
* mold

Mrs. Fnarf is much more responsible than me and has these:

* jumper cables
* window scraper
* tire pressure gauge
* tire chains
* sun shade

some great answers, but I always keep a set of chains on Besty starting in November until March...it sucks getting stuck downtown on the one time a year it starts to snow at 4 pm (remember last winter?)

A sonic screw driver is always good to have as well as a yo-yo, cricket ball, and psychic paper.

- LED flashlight: for when it gets dark at 4pm

- Window ice scraper: because the 1 time you need it every other year, you are really glad you have it

- A microfiber cloth: to clean your glasses and smartphone when getting in from the rain

- An outdoor blanket: surprisingly useful to keep around for the spur of the moment day/eve in the park

- A pair of work/utility gloves: your fingers will thank you when you change a flat tire in the cold rain

edit/bonus 6th: A bottle opener: because we're beer snobs and nothing worth drinking is twist off

Fnarf covered a lot of good ones like a Starbucks gift card and jumper cables, but I'd never carry tire chains -- overkill.

*Sunglasses
*A well-stocked iPod (for traffic)
*A punch card for your favorite coffee shack
*Reusable shopping bags in the PASSENGER SEAT where you will not forget them AGAIN, goddammit.
*Emergency item stash (flashlight, jumper cables, glass scraper, and a litterbag so you don't get a second ticket when pulled over).

Utilikilt, fedora, smartphone (to check while driving), travel mug, tangled rat-king of mobile device and iPad chargers.

There's really only one thing you MUST have... a Bon Scott-era AC/DC album. That will see you through anything, including this bullshit tunnel.

Battery jumper pack or jumper cables (and/or phone number for a well-regarded towing company if you dong have AAA)

Rain gear of some sort - waterproof windbreaker, umbrella, etc.

Window squeegee/scraper tool - about 4" across and they sell them in the automotive section at Fred Meyer

Garbage bag (did you know legally you are required to have one in the car?)

Reusable grocery bags

My car is also loaded with mom essentials such as: wet wipes; spare changes of clothing; a blanket to sit on at the park; sometimes I stash granola bars or dried fruit for an emergency snack; an extra baby carrier and a stroller.

1-- dollar coins
2-- tools
3-- blunt
4-- bottle of miracle bubbles
5-- costco case of naturevalley honey&oat granola bars