Motel One offers stylish rooms and good breakfast for cheap prices (starting at less than 60€/night).
There are three Motel Ones in Frankfurt. I stayed on Hanauer Landstrasse, which looks like an abandoned industrial area but is actually a hot spot of restaurants, clubs, and design shops. Tram 11 stops in front of the hotel and takes you to the central railway station in 15 minutes. The hotel has wi-fi in all rooms, but it's only free in the lobby - which results in the lobby being crowded with hip travellers lounging on the comfy sofas with their MacBooks.