It may say "Woodlawn Tap" on the wall, but anybody who's been there knows to call it Jimmy's. This is a venerable Hyde Park institution, and should probably be visited at least once for that reason alone.
In a study done by The Reader a few months ago, Jimmy's was found to have far and away the worst air quality in their entire sample of Chicago bars. I love Jimmy's, but I find it completely intolerable at night. In my opinion, the time to go is in the late afternoon, or for Bears games. In the afternoon, some sunlight comes in, the bartenders are happy to chat, and an interesting collection of regulars assemble after/during work. At night, the U of C students come in droves, and it's impossible to breath (I say this as a smoker!) or have a conversation with your friends.
The $2.75 pints of Leinie's Red and the $6 Bombay Sapphire martini's are the best deals here. The jazz set in the back room on Sunday evenings is fun and free.