Maybe give Sergio's a try. They get some pretty good (and some really terrible)
singers in there on Saturdays. I've been there on a lot of Saturday nights and it's pretty fun. They do it other nights too, but I know Saturdays for sure.
Another one is BJ Clanceys just down the road, but that's pretty much an older crowd just sippin' their drinks and seems pretty dull.
They do Karaoke some nights at Screwballs in Oshkosh (downtown on Main) but that place is pretty much a dive.