s5e10 "Dust to Dust"

That's Harrison Fjord!

s1e9 "Casino Royale"

 

That's Cameron from Ferris Buelher! Can you spot him?

s5e11 "Hands On"

 

That's Joe Piscopo!

s1e20 "Tribblets"

 

That's Lesbie Neilson!

s3e11 "Back in Time"

Where do I know this guy from?

s5e3 "Ceberus"

 That's fuckin' James Cromwell!

Or is it Freeman Dyson?

s3e19 "The Ocelot Equinox"

 

That's Kevin Nealon!

s5e1 "Q and A"

 

The alien Q is the airline traffic controller dad from AMC's Breaking Bad!

Update: John de Lancie

s4e22 "Who Watches the Watchers"

 

This guy's on the show on all the time! Who is this guy?

s2e7 "Zeitgeist"

That's T-bag from Prison Break!

s2e1 "Cats and Dogs"

That doctor Dr. Kelso from Scrubs

(hat tip to Chowdy)

s1e17 "Bird of Prey"

That's Knicks Cage!

s3e11 "Bread and Circuses"

Who is this jew? Woody Alien.

s1e4 "The King of Earth"

Will Forte from SNL and Tim and Eric!

s2e5 "The End of Time"

That's Don Salamanca from Breaking Bad!