I just completed watching the sixt season of 24, it was very good. And I’m not just saying that because lots of sci-fi actors popped up in it 
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)
