I'm a huge fan of 2Kolegas or 两个好朋友 as it's known in Chinese. It's a no-frills dive bar hidden within a drive-in-cinema compound on Liangmaqiao Road in the city's north east, and it's got one of the biggest - if not THE biggest - outdoor lawn for partying in Beijing. What's so great about it is the fact that it's so laid back and unpretentious, plus it's got a really chilled out summer vibe going on, with chuanrs being grilled outside throughout the night and rock or ska being belted out live on-stage inside. Sometimes, DJs even perform outside on the lawn creating a music festival atmosphere. The parties usually last until the next morning and quite often you'll be treated to unannounced performances by local musicians who just come to jam when all the official bands have finished. The drinks are reasonably priced too, but don't expect clean toilets!
http://www.2kolegas.com/

Other good venues include Jiangjinjiu (http://www.jiangjinjiu.cn/) just behind the drum tower. They put on intimate live gigs with a heavy emphasis on ethnic folk. Another good one in the area is Jianghu, just off Nanluoguxiang. The venue itself is pretty snug and good for catching an intimate folk show.

Also, don't forget about D22, the over-hyped and (annoyingly) self-proclaimed CBGB's of Beijing. I've personally been to the legendary CBGB club in New York when it still existed and I must say D22 doesn't even come close. Either way though, if you happen to be in the wudaokou area and crave live music then D22 is the place to go. http://www.d-22.cn/
Oh, and Wudaokou metal heads should go to Club 13, though I haven't heard of anyone going there in recent years.

Lastly, new venues on the block include Mako Livehouse and One Club, though they're a bit of a trek if you don't live in the Shuangjing area.