There are of course restaurants all over Frankfurt. One notable area for dining may be what is locally known as the Fressgass (a literal translation would be "eating road"). The correct name of this street is Grosse Bockenheimer Strasse. As the nickname implies, the Fressgass features many cafes, restaurant, and delicatessen food stores. It's a popular area to dine after the daily shopping. Take the subway to station Hauptwache or Alte Oper. In late May to early June (exact dates vary each year), the Fressgass Fest takes place with food stands, cheap beer and live music.
If you are looking for an in-depth paper-based restaurant guide, a popular publication is Frankfurt Geht Aus (Frankfurt is going out), a magazine style dining guide of the city. It can be bought for ?4.80 at many kiosks and book stores, or at the Tourism Information at the central station.