It doesn't get much more "Expanded Universe" than the 501st Legion – the one instance where the universe expands into our own. Odds are, if you've ever seen fans dressed as stormtroopers at conventions, they're among the 10,000-plus members of the biggest organized costuming club in the world. Back when "Vader's Fist" was only about one-third as large, Lucasfilm performed one of the coolest interactions in the history of fandom by officially indoctrinating the 501st Legion into Star Wars canon in Revenge of the Sith.
We never heard them referred to as the 501st in the film, but that's where Battlefront II picked up the slack. Beginning in the Clone Wars, the game's single-player campaign took us through the history of the organization – including the newly christened (still Christensen'ed) Darth Vader's storming of the Jedi Temple that we saw in the film.
It was an amazing tribute to fandom at its finest.