The best karaoke joints are the ones where almost nobody is good. They're places where not only do mediocre singers sparkle, but also where everybody feels comfortable getting involved. That's pretty much the M.O. at Jack's Club, a divey bar near the border of the Mission and Potrero Hill, on Thursday nights. At the corner of the bar, the effortlessly sexy DJ Purple plays backup saxophone to a wide selection of oldies, '80s, classic rock, hip-hop, and recent favorites. There is no stage, and it seems almost everyone feels entitled to contribute to any given song, so it's pretty hard to go wrong. But if you do, don't worry. Everybody is so wasted by the $2 Pabst Blue Ribbon that no one will even notice when you botch a lyric. "It's not fair/to deny me/the cross-eyed bear that you gave to me," a girl with hipster bangs screamed out not too long ago, sounding just slightly like Alanis Morissette. Ahem. That's "cross I bear," sweetie; you oughta know.