We don't really have a historical friendship club - not like Scheisse and Nurnburg, or Karlsruhe and Hertha. Having said that, we have recently struck up a really strong bond with Freiburg.
In last season's German cup final (where we battered FC Bayern ;o)) the Freiburg youth team were playing against Hertha in the German Youth cup final before the game, so all the Borussia fans went in and supported them. It was crazy, no Hertha fans, in their own ground, and 40,000 Dortmund fans cheering for Freiburg! The Freiburg first team coach then came and stood with the Borussia fans for the final.
So, what is with the we're not really here song guys?