TDSL is a variant of ADSL
made by the german Telekom
. They offer it mainly in big cities. It has a 768kbit downstream and a 128 kbit upstream. The TDSL itself is pretty cheap as you pay round about 5-7$ for it, but you also have to pay for the internet connection itself (of course by its subsidiary T-Online
You can combine TDSL with other providers ,too but this is not very useful as it is only round about 5-10$ cheaper but you have a download limit of 500 mb (which is not very much if you can download 100 mb in one hour).
The TDSL flatrate
costs 25$ and you have AFAIK no download limit (at least I have never managed to reach any and I download very much).