If you're heading up to Napa from the Bay Area, Cakebread Cellars is a great first stop, in Rutherford.
This small family winery has a small distribution. Tastings are $5 and you get to keep the glass. The wines are excellent and the tasting room is small, meaning it can get packed at times. But don't let that deter you from making a stop here, you'll be happy you did!