I was just on my favourite movie website -- good ol' IMDB (which incidentally was mentioned in a book I read recently called Aphrodite by Russell Andrews!) -- anyhoo I was just browsing and did a search for "ER" that great american show about the Emergency Room at a hospital in Chicago -- series written by Michael Crichton (of Jurassic Park fame).
They had so many people on that show as guest stars it was amazing. If you were ever wondering who guest-starred when click here. It's a list of the episodes in order with their guest stars and character names. And being the wonder that it is, IMDB have linked all the guest star names to their own actor pages so you can click them and see what else the person was in!!