Well-known guest stars:
Peter Berg (Noah, "Masquerade," "Snowman")
David Carradine ("Countdown")
Faye Dunaway (Ariana Kane, "The Abduction," "A Higher Echelon," "The Getaway")
Rutger Hauer (Geiger, "Phase One")
Ethan Hawke (Agent Lennox, "Double Agent")
Roger Moore (have to look it up)
Christian Slater (Neil Caplan, "A Free Agent," "Endgame")
Quentin Tarantino ("The Box," parts 1 and 2)
I'd say Terry O'Quinn, but he's almost a regular! Will add to this . . .