Just came back from business trip to Dusseldorf. Found two good places in the Old Town area: Pinnochio's (good Italian food, decent prices, good service, tiny place--go early), and Op de Eck (try the house cocktail with passionfruit juice, pricier, sophisticated, not typical stuff).
If you just want to swill some beer and dive into bratwurst and kraut, try Ham-Ham. Sorry that I am no good with addresses-- we simply took the tram to the Heinrich Heine stop, got off and walked. It isn't that big a place!