Cascade Cycling Club: If you want to ride STP or RSVP or any of the other rides in the west side, start here. This club is amazing; great rides, well organized, very popular and they fill up fast. Join up to get a registration discount and advance registration notice before it's released to the unwashed masses.
Puget Sound Area: Check out Classic Cycles calendar of events. They're on Bainbridge Island and have a SPECTACULAR bicycle museum...worth the ferry ride if you have the time.