Watching 15-20 year old shows is great. I'm about 18 episodes through season 2 of Law & Order, and have so far spotted early appearances from William H. Macy, Maura Tierney, Maria Pitillo, Lewis Black, Dylan Baker, Luis Guzman, Geoff Pierson, and a young Kelli Williams from Lie To Me and Judy Reyes from Scrubs. Plus Jerry Stiller. Legend.
Apparently there was also a very young Michelle Trachtenburg in one episode, but I didn't spot that.