Springfield is the setting for all of The Simpson's games and the hit television show. Nobody knows exactly where Springfield is.
Springfield is meant to represent anytown U.S.A. It has no fixed location and the city can often change locations based on the plot of the needs of the Game/TV Show.
Springfield's geography is often impossible, as West Springfield is the shape of Texas, and three times its size. Springfield also has equal driving distance to both oceans (only a couple of hours drive to each, maximum), and borders states that are nowhere near each other.
The most commonly accepted explanation given by show producers that is closest to "canon" is that it is in a fictional state. A recurring name given for this state is North Tacoma/Takoma (named after the neighborhood in Tacoma, Washington), as given by animator/director David Silverman; however no other member of the staff has given this name.