Budget

Jade, Moselstrasse 25 (close to Hauptbahnhof, 2 blocks down Kaiserstrasse, then right, it's the one on the left). Small Chinese place with an international crowd. Excellent food served in enormous portions for a very cheap price. The menu is in Chinese, English and German. Very friendly service.
Pizza Monte Carlo, Diesterwegstrasse 2 (corner Schweizer Platz), Daily 5:30PM-11PM (open 1st - 15th of every month only). A small pizza place in central Sachsenhausen, highly recommended as long as you intend to take your food home (and most people do), but it does have room for 2-3 people to sit. The pizza is handmade and very tasty. They also serve pasta and some salads. ?4-?6.
Bizim Döner, Train station Frankfurt-Griesheim (Lines S1 and S2 direction Frankfurt-Höchst). A small suburban shop with fantastic kebab. ?2.50-?10.
Pizzeria Charly Braun, Röderbergweg 121, Frankfurt-Ostend (two blocks south of Habsburgerallee U-Bahn station), +49 069 49 29 41. M-F 10:30AM-10PM, closed Sundays. A small place with cheap and excellent pizza. Pasta and salad also served. Locals usually call in an order and pick up in twenty minutes. ?3.60-?7.10 (depending on size).
Best Worscht in Town: place where you can try "Curry-Wurst", which is a sliced beef sausage served with ketchup and curry spices, and considered one of the most popular German fast food products. The Best Worscht in Town chain is special in that it also serves extremely spicy variants, using some of the hottest spices available from different parts of the world. There have even been televised hot sausage eating competitions with on-site medical staff to take care of the participants. Stores can be found on Berger Strasse, in the Nordwestzentrum, and other locations. These are just stands without any seating.
Im Herzen Afrikas, Gutleutstraße 13, Eastern African cuisine, you sit on the floor in sand. Reserving in advance recommended.
7 Bello, Niddastrasse 82 ,(close to the Hauptbahnhof) +49 069 236099. Probably one of the best italian restaurants in Frankfurt.
Cube, Leopoldstr. , [0] 69 / 4856
Bodega Dali, Breisacherstr. , [0] 69 / 97599-25
Il Borgo, Tiergarten, [0] 69 / 1369
Drugstore, Kumlbacher Str., [0] 69 / 7561
Golden Tweenis, Lindenweg, [0] 69 / 20571
Nordsee, Allers Weg, [0] 69 / 7363
Yum 2 take, Lorchheimer-Str, [0] 69 / 24807
nastraj, Weinstr. , [0] 69 / 21091
Restaurand Artemis, Schützenstr. , [0] 69 / 1536
nastraj, Leopoldstr. , [0] 69 / 24766
Nordsee, Untermarkt, [0] 69 / 2537
Kilombo, Luisenstr., [0] 69 / 16969

Mid-Range

Zum gemalten Haus, Schweizer Strasse 67 (Sachsenhausen): restaurant serving traditional local dishes, located in a building with painted facades
Leib & Seele, Kornmarkt 11 (near Hauptwache). Traditional German cuisine in a convenient location right in the centre.
Suvadee, Baumweg 19/Steinweg 7: in case you want a change from German cuisine: a very good Thai restaurant with very reasonable prices.

Splurge

Aquapazza, Westendplatz 42, ? +49 69 172 028 (ristoranteaquapazza@yahoo.com, fax: +49 69 972 06 440). Italian fish restaurant with option of alfresco dining under a pergola. Good, fresh fish dishes, but pricey.  edit
Orfeo's Erben, Hamburger Allee 45, +49 69 707 69 100. M-F noon-3PM and 5PM-1AM, Sa 5PM-1AM, Su 5PM-10PM. One of the most popular restaurants.
Haus Wertheym, Fahrtor 1, Am Römerberg, +49 69 28 14 32. Historic restaurant with excellent German cuisine. Reservations essential.
Surf-n-Turf Steakhouse, Grunebergweg 95 (corner of Siebert-Liebigstrasse), +49 69 722122. M-F 12AM-3PM, M-Sa 6PM-midnight. This is probably the best steakhouse in Germany. ?30-?50.
Taj Mahal, Schweizer Strasse 28, +49 69 620240. M-Su 11:30AM-2:30PM and 6PM-11PM. Indian-Pakistani food. Decoration is rather a bit overdone. Good service, very good food, prices a little high. Overall recommended. ?30+.
Emma Metzler, Schaumainkai 17, +49 69 6199 5906. Tu-Sa noon-4:30PM and 6PM-midnight, Su noon-6PM, M closed. A relaxed international-local cuisine with great service and changing menu. Set in the Bauhaus-Style Museum für Angewandte Kunst (Museum of Applied Arts), close to the Main river, but with no view. Quite recommendable. ?40.
Eatery, Neugasse, +49 69 / 793
Little India, Alte Gasse, +49 69 / 29903
Kairo, Leisberg, +49 69 / 8592
Taste Of Darkness at Dialogmuseum: experience eating in the dark without being able to see a thing (slightly expensive though)
Gargantua: Liebigstr. 47. Considered one of the best restaurants in Frankfurt according to Zagat.