Angie and Isis - I'm chuckling because Isis could have written my answers! And Rosee, too!
I mostly ride to work, so I'm gearing my answers to that.
I use a Timbuktu messenger bag for my laptop and work papers because it's TOTALLY waterproof - it's gear, but it's so functional that I'll never give it up. I've been in downpours and gotten home and my precious laptop is dryer than dry. When I have spillover (e.g. because I'm carrying shoes, lunch, etc.), I use a pannier that is also waterproof. My handbag (I now own 3!) fits INSIDE either of them, so it can't be too large. Just like Isis, I use a smaller cross-body. When I get to work I can pull it out to run to meetings/lunch when I don't need my computer.
In summer, I mostly just bike in what I wear. If I have a dress on, I often pull biking shorts or leggings on underneath for the ride for modesty (e.g., when my dress flips up over my face). I can't bike in a pencil skirt, so that gets rolled up and put into my pannier or or messenger bag (the Halogen ones are great - NO WRINKLES!). If it's hot, I will strip down to one layer and the cardi for the cold office goes in the bag. My other big exception is that I try very hard not to bike in white jeans! I've made that mistake already. I just swap them out for bike shorts and roll them up into my bag. I got bike grease on my new white bootcuts the first time I biked in them : (. I was able to get it out, but I don't want to risk it again. Like the others, I put straps (or rubber bands) around my ankles when I'm wearing bootcuts or flares.
In winter in MN, it's a whole different ballgame. I wear snow boots, goretex rain pants, my down parka, a headband+hat, overmits over fleece mittens, etc, etc. I leave my shoes at work (or put them in my pannier). Since I have my down parka (my puffer), I can just wear that around campus without all of the extra gear.
For rain, I'm like Isis - rainboots, rainpants, raincoat (with the hood up and the helmet over it). My shoes are in my bag or pannier.
Fall and spring (and I'm imagining much of the weather in Seattle) are heavenly, as I don't have to do as much swapping out of gear. That said, this morning I wore my regular clothes and the short black Gallery trench that Angie recommended this past summer (which, I might add, is PERFECT for biking - it's like a fashion-forward windbreaker as it definitely blocks the wind without being too hot AND it rolls up into my bag nice and compactly - THANKS, Angie!) and my headband and warm windproof gloves. The gloves and headband are definitely GEAR. There's no way to not include SOME gear when the weather gets colder.
Since I've got the podium, PLEASE WEAR A HELMET! I know that they don't in Europe, but European drivers are TRAINED to be aware of cyclists. American drivers are not. I've been hit twice by cars, once ending up in the ER. My helmet has saved my LIFE, and I can't stress enough how important it is to wear them. I know that helmet head is a problem, but not when you're dead! Just be sure to shoot your WIW photos in the MORNING before your hair is a wreck ; ).
Here's my messenger bag:
http://www.timbuk2.com/tb2/cus.....;uid=61636
Here's my pannier:
http://www.banjobrothers.com/p.....f-pannier/