Pandora is a fun bar/lounge in downtown SF (pretty close to Powell BART). On Friday nights karaoke begins at 6pm with happy hour prices between 6-8pm for well drinks. As far as free karaoke lounges go, Pandora has a great atmosphere and typically brings in a supportive crowd.
Maybe you have other plans on a Friday night? Stop in a for a drink, and maybe a song, on your way to another event. Typically early Friday nights are pretty tame, which = lots of singing! Later in the night things amp up, bringing more of a party-like vibe to karaoke.
I look forward to seeing old friends and making new ones!