In its heart of hearts, Sonoma remains a farm town, which means early to bed, early to rise. Except for late night drinking around the plaza there isn't much to do at night. Little Switzerland, about 2 miles from the plaza sometimes has music on weekend nights. This is a very local place. The selection is eclectic varying from very good zydeco, to jazz, country, swing and even polka. Mostly this is a place to dance. Beer is good. There is some wine, but I advise avoiding the food altogether. Cost is always reasonable.
Dress Code: Remarkably informal
Actually, you don't have to wait until night to go to Murphy's; they serve lunch also. It's a nice place to have a plate full of Sliders (mini-hamburgers or veggie burgers) and a pint. Nice selection of beers and ales, and there's often live music in the evening.
Murphy's was totally renovated in early 2011 and now has a longer, more comfortable bar, and a heated patio. Families are welcome.
Be sure to stop by the best bar in Sonoma while you're in town -- and the oldest establishment on the Plaza. Steiners, founded in 1927, has an excellent selection of liquor and beer and a well-rounded local wine list. Bartenders are super friendly and pour generously, but the bar has remained reasonably priced. There is usually several sports games on and there are two pool tables. A digital juke box and occasional live music on weekend nights make it a great hang out. Friendly locals make up the daytime crowd, while 20-somethings and tourists pack the place at night. Cash only, but ATM on location. And don't miss happy hour from 6am to 10am -- hey, it's 5 o'clock somewhere, right?