Oh..... a couple ......dozen.....
possible spoilers
Don S. Davis (Beyond the Sea & others)
Amanda Tapping (Avatar)
Tobin Bell (Omac in SG 'Enigma', in Patient X)
Chris Owens (Armin in 'Secrets', played Agent Spender)
Megan Leitch (young Linnea, played Samantha Mulder)
Steven Williams (General Vidrine, played Mr X)
Steve Makaj (Makepeace, in Redux and Redux 2)
Robert Wisden (Lt Col Samuels, in Pusher and Kitsunegari)
Tom McBeath (Maybourne, in Tesos Dos Bichos)
Tamsin Kelsey (from Thor's Hammer, in Jersey Devil)
Steve Bacic (Maj. Colburn, in Folie a Deux)
Hrothgar Matthews (one of the Drs in Prodigy, in Talitha Cumi and Jersey Devil)
Brian Markinson (the alien in Scorched Earth, in Folie a Deux)
Marshall Teague (Frank Cromwell, in Fallen Angel)
Colin Cunningham (Lt Simmons, in Wetwired)
Willie Garson (Marty, in Goldberg Variations)
Ron Halder (Cronos, in Patient X)
Jay Avocone in 2 in Season 8, one which has aired, Per Manum, and one which hasn't aired and has no title yet
Phew. I'm tired.... there's probably more . . . .