Cool little fact:
Alexis Denisof (Wesley from "Angel") was The Other in "The Avengers". You know, the boss alien that was talking to Loki, and later Thanos at the end of the movie. He knew about the Thanos shyt months in advance and Joss told him

"you better not spoil that shyt".
He's also married to Willow (Alyson Hannigan) in real life.
And obviously Amy Acker was in "Cabin in the Woods" recently.
Like someone said J. August Richards (Charles Gunn) is gonna be in "SHIELD".
Lastly, after "Avengers" Joss Whedon did a modern remake of Shakespeare's "Much Ado About Nothing" with a bunch of people from "Buffy" and "Angel". It got rave reviews apparently:
One thing I like about Whedon is he always feeds his people, even if they aren't stars.
Fred.