This combination youth hostel and family hotel offers clean, inexpensive, and very central accommodations in what is usually a pricey city. It's on the river in Sachsenhausen, directly across from downtown. A stay in a 10-bed dormitory room will cost you €18 a night (€23.50 if you're over 27), or have a private room with bath for a reasonable €42). That price includes breakfast, and a lunch or dinner buffet is yours for €7. In warm weather, you can eat in the outdoorgarden.
You must be in by 2 am or get a key.