When I lived there (I'm now down in Salem), the Buffalo Gap on Macadam had some folkie nights. I don't remember any open-mike, though. My brother hangs out at the Cafe Lena and loves it. You might also check out the McMenamin's family of pubs; several of their venues feature live music of many kinds, the more eccentric the better.

And get a subscription to Willamette Week. It's a required survival manual, and the cartoons are great.