Some are very active, some less so. I am part of the N. Seattle Homebrew club that generally meets once a month. Meetings range in size from probably 10 or so all the way up to 30+. We generally meet either at a brewery or someone's house (we've had meetings at Pike, Georgetown, Fremont, off the top of my head). Our organization is fairly loose (no dues, no officers, etc.) but it works.
Rat City Homebrewers is a new club that recently formed. West Seattle and Burien area, but open to all. Meets second Thursday of the month at Big Al Brewing @ 7. Pretty informal now, but expect more structure in the coming months as we get more cohesive.