Answer:
Well known actors who have appeard in Law & Order SVU:
Barry Bostwick
Blair Brown
Betty Buckley
Didi Conn
Beverly D'Angelo
Pam Grier
Marcia Gay Harden
Mariette Hartley
Jeremy Irons
Robert Klein
Shirley Knight
Christine Lahti
Judith Light
Ron Leibman
Ludacris
Mary Stuart Masterson
Marlee Matlin
Bebe Neuwirth
Jerry Orbach*
Annie Potts
CCH Pounder
Peter Riegert
Sharon Stone
Marlo Thomas
Sam Waterson*
Diane Weist*
I know that I've missed a favorite of yours, please add them to the list (in alpha order please).
*These were included, not because they are well known from Law & Order, but because they were stars before they appeared in the series.