Intercityhotel is always a reliable stand-by. They are usually colocated with the main train stations, give free internet access and a free public transportation ticket.

I've stayed at the one at the Frankfurt Hbf. Small, sober room with a comfortable enough bed and a small bathroom with shower. Its all you really need, cheap enough (as long as there are no conventions going on), and very central. It is also convenient to bring food from the station, or there are plenty of restaurants nearby.

The big US chain hotels usually charge something in the neighborhood of 20 euro/night for internet alone.