You should search the recommended resources threads, this is a popular theme!
My kids are older now, but off the top of my head, an app called stack the states, the national geographic bee site and app, the book scrambled states of America, a game based on the book of the same name (there are actually 2 games, possibly more, my kids liked the one with cards and maps better than the dice-based game), "where in the US is Carmen Sandiego" (this was software, but from several years ago now, so don't know if it's still around), lots of exploring with google earth, (found a random book with a scavenger hunt done via google earth, but can't recall the name; obviously, this one is not just US geography).