It looks like USSOCCER.COM has changed their site around a bit, but there are still tons of valuable resources to be found there. I recommend selecting the Referees link on the menu bar at the top of the ussoccer.com page, then the Laws of the Game link on the left sidebar. There, you will find Referee Education Resources Download at the bottom of the page, and that's where you can find videos, interactive offside quizzes, etc. The Resource Center still has the online training course, but the additional stuff has moved to the location I listed above.
Of particular interest are the videos, books, and FIFA Resources sections. I've found numerous items that have helped me raise my game, I feel, and become a better referee. I hope you will take some time to poke around the site and watch some videos, read some books, and raise your game, too!