The following actors and actresses have appeared on Seinfeld and then went on to star on other shows.
Courteney Cox was Jerry's fake wife to get a dry cleaning discount (Friends)
Janes Leeves was Marla the Virgin (Frasier)
Larry Hankins who was TV Kramer in The Pilot (Mr Heckles on Friends)
Jeremy Piven who was TV George (Ellen)
David James Elliot was a furniture mover who was anti-abortion so Elaine had to break up with him (JAG)
Lee Garlington who was Clare the waitress in The Seinfeld Chronicles (Townies)
French Stewart was the theater employee who told Elaine Jake Jarmel was in the hospital and then Elaine stops off for Jujy Fruits (Third Rock)
Nick Bakay was Elaine's boyfriend who thought she stunk in The Smelly Car (voice of Salem the cat on (Sabrina)
Kathy Griffin who was the woman who thought Jerry was the devil (Suddenly Susan)
Michelle Forbes took credit for George's big salad (Homicide)
Jami Gertz didn't have a square to spare(ER)
Dan Cortese who got messed up rock climbing with Kramer and George (Single Guy and Veronica's Closet)
Vicki Lewis was George's Secretary (Newsradio)
Wendy Malick was a physical therapist who charged George for missing an appt. then skipped an appt. with him to go skiing with Elaine in The Kiss Hello (Just Shoot Me)
John Favreau was Eric the clown who put out the fire with his big shoe in The Fire (Monica's rich boyfriend on Friends and also, Swingers)
Teri Hatcher- are they real? (Lois and Clark)
Kathy Kinney was part of a mob when George parked in a handicap spot (Drew Carey)
Lisa Edelstein who feels full after the risotto (Relativity)
Christa Miller had her material felt by George when he became a bra salesman and also was in Elaine's art class in another ep and didn't care what George looked like (Drew Carey)
Dedee Pfeiffer "I'm Victoria. Hi" In The Opposite (Cybill)
Arman Shimerman who was Kramer's Caddy when he sued the bra-less wonder (Star Trek Deep Space Nine)
Brad Garrett who was the mechanic that took Jerry's car (Everybody Loves Raymond)
Paula Marshall "outed" Jerry and George (Chicago Sons)
Jessica Hecht was George's girlfriend when he rented the movie Breakfast at Tiffany's instead of reading the book. Also, was his girlfriend in The Gymnast (Ross's ex-wife's lesbian lover on Friends and also played a different character on The Single Guy)
Maggie Jacobson, now Wheeler was fixed up with George in The Fix-Up (Janice on Friends)
Danny Woodburn was the short actor standing in for little kids (Conan)
Stephen Hytner as Kenny Bania (Working)
The actor who bought Mr Costanza's old clothes at Rudy's (Can't think of his name) (Building super on Friends who danced with Joey)
Did I leave off anyone?