If you look at tv shows you can see other tv stars before the made it big on their own shows.
Examples:
on Seinfeld:
Megan Mullaly played George's girlfriend now she is Karen on Will and Grace

Debra Messing played Jerry's girlfriend now she Plays Grace in Will and Grace.

Wenie Malick played Elaine's therapist friend now she plays Nina in Just Shoot Me.

Some other guest appearances of note:

1. Alley Mills had a guest appearance on "The Waltons" in the episode entitled "The Obsession". According to the website www.the-waltons.com "Mary Ellen abuse[d] prescription drugs in an attempt to study long hours for her nurse's exam." Alley, of course, is perhaps most famous as Kevin Arnold's mother on "The Wonder Years".

2. Jason Hervey played a guy named Charlie on "Diff'rent Strokes" during the last season or two. Jason, of course, is perhaps best known for his role as Kevin's brother Wayne on "The Wonder Years".

Also: Was it "Bewitched" that Maureen McCormick was on in one episode before becoming most famous for her role as Marcia Brady?

I recall seeing Barbara Eden on the I love Lucy show before she played Jeannie In I dream of Jeannie.

I also recall Loni Anderson on Three's Company before she hit big.

Harry Anderson guest starred on Cheers before Starring in night court.

Seinfeld used a ton of folks who went on to other shows. I have a theory that when a person shows up on more than one NBC show, he/she will be in a sitcom the following Fall.

Case in point, Lauren Graham appeared on a few NBC shows, like "Newsradio" and "Caroline In The City." A short time later she had a co-starring role on a short lived NBC show called "Conrad Bloom" and then went on to "MYOB" then "Gilmore Girls."

Lisa Kudrow guest starred on an episode of Cheers that Ithink was titled, "Two girls for every boyd"And now she is on friends

Ann Schreden (sp?), the mom from "ALF", was in around 5 episodes of "Three's Company". Joanna Kerns, mother from "Growing Pains", was in two episodes, one in 1979 and one in 1983. Ellen Travolta of "Charle's in Charge" appeared on it, along with Brian Robbins and Jeanetta Arnette, both castmembers of "Head of the Class". Ilene Graff was in an episode of TC too, went on to "Mr.Belvedere". "Gimme a Break"'s Lauri Hendler was in an episode also. David Ruprecht is the host of "Supermarket Sweep" and Eleanor Mondale was (or is?) host of "CBS This Morning". All appeared on TC. lol