I second the Tchibo vote. I moved to Germany on a short term basis recently and signed up with these guys.
It's completely prepaid, but you can sign up for a monthly "plan" which automatically deducts money each month in exchange for a data allowance. You can turn plans on/off by either texting a certain number and possibly via web interface too.
I signed up for the 5GB plan (shaped to 64kbit after) and it only costs €20 a month, which seems a fair bit better than what most providers offer for heavy data users. The coverage isn't amazing compared to t-mobile but it's not too awful if you're not rural and for standard calls o2 has a roaming agreement with t-mobile as well. I get very good data rates and few dropouts using Tchibo in Augsburg as my sole internet connection aka. fairly heavy usage.
For anyone wondering why on Earth anyone would get 5GB of 3G data, the answer is tethering + living somewhere without wired internet.