1. Dr. Mae Jemison - American Astronaut
Not only is this one of my favorite cameos, but it also occurred during one of my favorite episodes of Star Trek: The Next Generation. Dr. Mae Jemison, an astronaut with the United States space program, plays the role of Lt. Palmer in Second Chances. This is the episode where Will Riker discovers that a double of himself, created by a transporter beam anomaly, was marooned on a deserted planet eight years ago. Prior to the airing of this episode, I had often wondered why we never saw duplicate beings created by the transporter (although a physical, but not mental, duplicate of Kirk was created in the TOS episode entitled The Enemy Within). Surely some lonely transporter officer at some remote Federation outpost has perused his transporter memory banks and created a duplicate from one of the apparently endless supply of gorgeous girls in the Star Trek universe. A really good transporter officer could even add the personality he wanted in his new love slave, a case for which can be made by watching the Tuvix episode of Star Trek: Voyager. Anyway, I think this cameo is one of the greatest just because Mae Jemison is the only person to appear in Star Trek who has actually BEEN in space - while still alive, of course. How cool is that?
So that's my list. What unexpected cameos did I miss? Let us know in the comments section below.