Heidelberg's Hauptstrasse is the longest (~1.5 kilometer) pedestrian mall in Europe- or at least it was when I was there. Lots of buskers. Some good, some not so. Good local music sceene as well. I used to host the open mike night in a cafe on the Marktplatz- when someone was good, we'd give 'em a flyer & invite them in to play. We'd pass the hat (literally a big-assed spangly sombrero) & pour 'em a couple of beers.
Shoot me a PM & I'll pass along a couple of names/venues for you.