One of the best places in Heidelberg to swill beer and enjoy good music is a little place called Destille. The bar is located on Unterestrasse in the old part of the city.
Most nights, the place is completely packed by 22:00. It's elbow-to-elbow, loud, smoky and hot. If you're sensitive to cigarette smoke, don't go. At 23:00, they close the door and windows to contain the noise, and you can almost see the smoke cloud start descending from the ceiling. I guess that's why not many Americans go there.
Across the street are a few other bars, and the overflow from all of them fills the street with partiers. The wait staff from each bar go nuts trying to figure out which glasses to collect, and I've walked by on Saturday mornings to see the staff returning glasses to the right bar from the previous night's festivities.