This article is from the ER FAQ, by Rose Cooper firstname.lastname@example.org with numerous contributions by others.
You can send your letters to:
Attn: [Actor's Name]
c/o Crichton TV
4000 Warner Brothers Blvd.
Production Building #1
Burbank, CA 91505
A user named Heidi (email@example.com) reportedly sent a
letter to Julianna Margulies using this address and got a response
and an autographed publicity picture back.