Mörfelder Strasse 113
The NH Hoteles (sic) chain operates hotels in Europe, Latin America, and Africa. The one at Frankfurt Airport is a professionally and efficiently run business hotel. My non-smoking en-suite (bath and toilet included) room, with a queen-sized bed, TV, and phone, came to €66. Breakfast is optional at €15, so I didn't bother.
The hotel runs a shuttle to the airport every 20 minutes from 0500 to 2345. It's a longish walk (maybe a couple of kilometers) which I wouldn't want to do with baggage, but the shuttle is quick.
There's a local bus stop across the street which will take you to the airport and to Kelsterbach. Both have S-bahn stations, but I found myself using the shuttle because it runs a lot more frequently. There's also a small pay parking lot in front of the hotel. I imagine you get a deal if you actually stay in the hotel.
Not much character, but I'd definitely stay there again as it's one of the least expensive options near the airport. An oil man from Alberta I met on the shuttle says he's stayed in most of the hotels near Frankfurt airport and this one is his favorite. I can see why.