These famous people guest starred on the show, in appearance order:

Dante Basco (three), Bobbie Eakes, Mark-Paul Gosselaar, Breckin Meyer, Dustin Diamond (twice), Adam Jeffries, Larenz Tate, Lindsay Price, Patrick Cronin, Maia Brewton (twice), Kelly Packard, Ben Savage, Soleil Moon Frye, Rick Hurst, Chad Allen, Jonathan Brandis, Carla Gugino, Alicia Silverstone, Brice Beckham, Seth Green (twice), Andrew Shue, Paul Gleason, Gordon Clapp, Monty Hall, John Pleshette (twice), A.J. Langer, Timothy Stack, Jean Speegle Howard, and Peter Spellos (twice).

Also Joseph Dammann.
He was in a 80's Christian series called McGee and me.
I read he was on Internet movie base.

Also, don't forget David Schwimmer....Karen's hubby, Michael.

akpark, according to IMDB.com, he was listed as a cast regular during the 91/92season, not as a guest star.