The outdoor concerts at this former squat are great. They also show movies during the summer.

Personally I don't care much for the bar part with all the live soccer transmissions, but some of the best indie concerts in Vienna have taken place here -- it's always worth checking their website to see who's playing. Beware, however, that it's often WAY overcrowded. (Examples of artists too popular for the Chelsea: Xiu Xiu, 13&God).

The Luftbad is a cozy, intimate, down-to-earth bar tucked away in a small street near the Apollo cinema. It has a little stage, which often hosts local funk/soul/acoustic/world music/jam/hip-hop bands.
I haven't spent a lot of time there, but I've always gotten the impression that everyone else in the room knows eachother personally. Although that's probably because we actually crashed a private birthday party the first time we wandered in. I gotta say, the cake was delicious.

Intimate, not too attractive little venue. Live shows are mostly punk, ska and similar styles, but they also host a bunch of indie clubs.