Robert Carlyle (Redemption)

24: Carl Benton
Star Gate Universe: Dr. Nicholas Rush

Tony Todd

24: General Benjamin Juma
Star Gate SG-1: Lord Haikon
Andromeda: Captain Fehdman Metis
Star Trek Voyager: Unnamed Hirogen hunter
Star Trek DS9: Jake Sisko as adult in The Visitor
Star Trek TNG/DS9: Kurn

John Billingsley

24: Michael Latham
Star Trek Enterprise: Dr. Phlox

Kurtwood Smith

24: Senator Blaine Mayer
Star Trek Voyager: Annorax in Year of Hell

Jeffrey Nordling

24: Larry Moss
Star Trek DS9: Thana Los in Past Prologue

Mark Kiely

24: Edward Vossler
Star Trek Voyager: Lasca in Non Sequitur

Nick Toth

24: Edwin Ross
Babylon 5: Exeter in The Deconstruction of Fallen Stars

Annie Wersching

24: Renee Walker (seriously hot FBI agent)
Star Trek Enterprise: Liana in Oasis

Colm Feore

24: Henry Tailor
Battlestar Galactica: President Adar (First episode only)

Bob Gunton

24: Ethan Kanin
Star Trek TNG: Capt. Benjamin Maxwell (in The Wounded)

Ryan Cutrona

24: Admiral John Smith
Babylon 5: Sgt. Major Plug in GROPOS

John Rosenfeld

24: Kevin Aldridge
Star Trek Enterprise: Mark Latrelle in Silent Enemy

It’s also worth noting that co-executive producers and writers of 24 Redemption and season 7 is none other than Brannon Braga and Manny Coto, noted writers/producers from Star Trek TNG / Voyager / Enterprise.

UPDATE: You may also want to check out a similar listing for season 7: Sci-fi actors in 24 season 7

I compiled a list of the characters in season 6 of 24 that I’ve also seen in science fiction shows:

James Morrison
24: Bill Buchanan (leader of CTU)
Space: Above and Beyond: Lt. Col. Tyrus Cassius McQueen

D.B Woodside
24: President Wayne Palmer
Buffy The Vampire Slayer: Principal Robin Wood

William Devane
24: James Heller
Stargate SG-1: President Henry Hayes

Gregory Itzin
24: former president Charles Logan
Star Trek DS9: Ilon Tandro
Star Trek Enterprise: Captain Sopek, Admiral Black

Roger R. Cross
24: Agent Curtis Manning
Star Trek Enterprise: Tret

James Cromwell
24: Phillip Bauer
Star Trek TNG: Prime Minister Nayrok
Star Trek DS9: Minister Hanok
Star Trek First Contact: Zefram Cochrane

Tzi Ma
24: Cheng Zhi
Star Trek TNG: unnamed Biomolecular Physiologist

Kari Matchett
24: Lisa Miller
Invasion: Dr. Mariel Underlay

Glenn Morshower
24: Aaron Pierce
Star Trek TNG: Ensign Burke, Administrator Orton
Star Trek Voayger: Mokra guard

Adoni Maropis
24: Abu Fayed
Command & Conquer Tiberian Sun: Hassan

Michael Shanks
24: Mark Bishop
Stargate SG-1: Daniel Jackson

Alexander Siddig
24: Hamri Al-Assad
Star Trek Deep Space Nine: Dr Julian Bashir

Also, I thought a FBI agent was played by Star Trek Voyager’s Robert Duncan McNeill, however the FBI agent was played by Scott William Winters, which just happened to look and sound very similar

Don’t tell me this could not be the same guy (12 years apart)