The only one I know of that goes till 8 is Boiler Room downtown, which runs a drinks special till nine.
However, there's a bunch that start at 9PM and run late.
Japanese restaurant Biwa has a highly confusing but highly delicious happy hour that runs from 5:30-6:30 but then starts up again from 9-10. Guild Public House on Burnside does happy hour after 11pm.
There's a couple good places for happy hour food all night on certain nights. Hungry Tiger 2 does $1 corndogs all Wednesday night and Hall of Records has half priced sandwiches all Monday night.
I'm embarrassed that this is all in my brain. Whatever, I'm cheap. Oh, and I should put in a plug for the Merc's happy hour iPhone app, Cocktail Compass, which lists local happy hours. I've never used it, though, since I don't have an iPhone.