The Danza: Chuck the Technician, played by Chuck Campbell. He got his name when Torri Higgenson accidentally called him by his real name in a scene instead of his character's rank (Sergeant) and it was left in as a Sure, Why Not?.
It's rare for fans to refer to the Genii leader by his real name rather than "O'Brien".
Michael is often conflated with Trip Tucker, and so on.
Mylène Dinh-Robic, who played "Dr. Olivia Fawcett" in "The Listener" also played as one of the Athosian prisoners in episode "The Kindred".
Nicole de Boer played Dr. Alison Porter in the episode "Whispers". She's the one who played Ezri Dax on Star Trek: Deep Space Nine. A young Nicole de Boer had also previously appeared alongside David Hewlett in Cube
Mr. Woolsey will be familiar to Trekkies as the Doctor (no, not that one), and Dr. Keller was played by Jewel Staite, who was also Kaylee on Firefly.
Incidentally, Jewel Staite also played the Wraith child Ellia in the episode "Instinct" in heavy makeup for the very reason of allowing her to possibly come back later as another character.
Real-Life Relative: David Hewlett's sister Kate Hewlett plays Rodney McKay's sister Jeannie Miller. In "Hot Zone", McKay was orginally supposed to mention he had a brother, but David Hewlett asked if he could change the line to sister, in the hopes that he could get his real-life sister to star alongside him.
Throw It In: After Torri Higginson accidentally called the Gate Technician by his real name, "Chuck", the producers decided to keep it in and make it has official name, since he had not had one until that point.