There used to be a bus route that went from Seattle to Bellevue at 2:15 and 3:30 am, but it got shut down as part of recent budget cuts. :(
On the other hand, I can kind of see why. According to Metro's statistics, almost all riders on the 280 were using it as a mobile shelter instead of transportation to a specific point. (Judgment on whether that is good or bad is left as an exercise; I personally had no problem with it.)