I would add that "cheap" usually extends to some parts of the former West, namely Kreuzberg (quite a few startup/tech meetups there), Neukölln, and Wedding (not my personal favourite but definitely cheap), all of those used to be "in the shadow of the wall".

When we had a larger apartment and were trying to find people to rent one of the rooms for a few months, wg-gesucht.de was very useful, though that was a few years ago.