I've been watching Mission:Impossible Season 1 specifically to play spot-the-Trek-actor, and, man it has not disappointed. Nearly every episode in Mission Season 1 has someone from TOS Season 1 in it, which makes me think that guest actors were signed to do the Trek/Mission double (to save money at cash-strapped Desilu perhaps?).
Anyway, the most interesting thing for me so far was discovering that two Enterprise crew were actual IMF agents, not guest villains or whatever, but actually on the undercover team! The two actors to have the distinction of being both Starfleet officers and IMF agents are George Takei, and Eddie "Mr. Leslie" Paskey.