Recognizing M from the early Bond movies as the sergeant in The Third Man when he was much younger was a highlight for me.
Most recently, I recognized Danny Noonan on Homeland.
Clark Gregg plays Agent Coulson in the Marvel Cinematic Universe: Phase One movies and in Agents of SHIELD. I was shocked to go back and realize he also played an FBI agent on The West Wing.
He was an angel in one episode of Alfred Hitchcock Presents.