Upvote:0
AirBnB has quite a large selection for Berlin.
Upvote:2
A popular site for finding apartments in Germany is ImmobilienScout24. A competitor is Immowelt. Note that, normally, apartments are rented unfurnished.
If you are looking for furnished apartments for up to just some months, have a look at sites such as wg-gesucht.de or zwischenmiete.de. I wrote about that in an answer to a question about short term rent.