A few months ago, Peter McEntegart of SportsIllustrated.com had a fantastic post about actors whose careers were more or less launched by Seinfeld. I've been using his observations during conversational lulls ever since. A few highlights:
Peter Krause (left, at left), later of Sports Night and Six Feet Under, played Tim the Neo-Nazi in "The Limo" "“ the episode where George and Jerry played Colin O'Brien and Dylan Murphy to avoid the cab line.
Jon Favreau dressed up to play Eric, the clown George pushed out of the way (along with the women and children) during "The Fire."
Here's one that's not on the list "“ Jeremy Piven. Though his career was already picking up by the time he played Michael Barth, who played George Costanza on the Jerry pilot.
Plenty more to be found at McEntegart's original article, including Megan Mullaley, Michael Chiklis and Denise Richards.