Quite a few Doctor Who crossovers, including both Thomas Sangster and Harry Lloyd in the Human Nature/Famly of Blood two parter. Joe Dempsie being in The Doctor's Daughter (as well as in another show, The Fades). Mark Gatiss playing both several roles in Who (most notably Dr. Lazarus in The Lazarus Experiment) as well as writing several episodes. Liam Cunningham as the captain in Cold War. I'm sure I'm missing more. Roger Allam was also great in V for Vendetta and The Thick of It.