Starting on December 29, the state will start collecting electronic tolls on the SR-520 bridge between Seattle and Bellevue---meaning that if you cross the bridge on an even semi-regular basis, you'd better hustle to Fred Meyer, Safeway, Costco, or QFC (or go online) to get your "Good to Go" pass to cross the bridge.
If you don't have a pass, WSDOT cameras will take a photo of your license plate and send you a bill for the toll, which will be $1.50 higher---each way---for cars without a pass. If you're driving a Zipcar, it's already equipped with a pass; the tolling charge will appear on your bill.
WSDOT has a list of FAQs---including "Is this really, truly going to happen?" (tolls were supposed to start a year ago, but the state's contractors ran into technical problems)---here.