For me a pannier set/ trunk for the back rack is essential to my commute. I chose the Topeak MTX system with the fold-out panniers; great system, sturdy, durable and easy to use.

The other necessary is a light, even if you think that your commute will be daytime only, you never know when the boss will stick you with a huge project at 4:30 that lands you on the road at 9 pm in the dark. I recently purchased the NiteRider Cherry Bomb (http://randomstreamoc.blogspot.com/2...ike-light.html) This thing is extremely visible, I use it even pre-dusk just to be careful. (It also comes in handy for early morning commuting in the fog, which I am sure you will experience in Seattle)

Enjoy your new lifestyle!

-J