If you've seen Joss Whedon's previouswork (or even non-Whedon genre TV), there's a lot of familiar faces on display.
The leader of the cult in "True Believer" (1x05) played the Terminator "Carter" in The Sarah Connor Chronicles, the actor who plays Zone in "Epitaph One" (1x13) gets a cameo in the same series as a human resistance fighter, and the pedophile kidnapper in "Echoes" (1x01) played the Terminator "Vick". And then Cameron showed up in "The Public Eye" (2x05) and "The Left Hand" (2x06). The actor who plays Clyde in "The Attic" also played the doctor who helped Cromartie regrow his skin in the first season of SCC, too. Adding the surreality of that, Bennett's assistant is the same actress who played the guidance counselor who interviewed Cameron at the halfway house in SCC's "Allison From Palmdale."
And the warden at the prison in "Meet Jane Doe" (2x07) is Aaron Pierce.
Patton Oswalt played an Internet tycoon named Joel Mynor in "Man on the Street" and "A Love Supreme." You may recognize him as the voice of Remy from Ratatouille, which is given a Shout Out in that Mynor's most famous creation is named Bouncy the Rat.