Morena Baccarin starred as Inara on Firefly, and recently was the Ori leader Adria on Stargate SG-1.

Jolene was, of course, the logical Vulcan Commander T'Pol on Star Trek: Enterprise.

aka Cordelia Chase from TV's "Angel"

Lexa Doig was the manifestation of the ship Andromeda on the series by the same name. She's also been a doctor on Stargate SG-1, among other sci-fi roles.

Erica Durance currently stars as Lois Lane on the CW's Superman TV series, Smallville.

The Cylon No. 6 on TV's Battlestar Galactica. By your command!

Natasha's playing a lawyer on Eli Stone now, but we all remember her as the alien from Species.

Battlestar Galactica's Cylon, Boomer

The X-Men's screaming blue meanie, Mystique.

Star Trek: Voyager's "7 of 9." Assimilate me.

Jewel Staite played the mechanic Kaylee on Firefly, and later Serenity. Currently she's the doctor on Stargate Atlantis.

Amanda Tapping is of course Samantha Carter on Stargate SG-1, and now Stargate Atlantis.