Jerry Orbach is my favorite non-living Broadway actor. If none of you have ever heard him sing, go to the library and get out The Fantasticks or Carnival soundtracks and hear him sing. He was just incredible!
I love how Law & Order takes Broadway actors to play bit parts in its episodes. I think it really enhances the show and creates great chemistry with main actors like Jerry Orbach and Jesse Martin who are also Broadway actors. And I almost flipped out of my chair when I saw Terrence Mann, my current favorite living Broadway actor's name on the guest star list a few weeks ago
Det. Lennie Briscoe: If I was kiddin' you, I'd be wearin' a fez and no pants.

Don't forget Judith Light, Dianne Weist, Bebe Neuwirth<< the incomparable RITA MORENO!!

I read somewhere that one of the things that young actors look forward to in NY is being on L&O in any kind of part. It has been a blessing to acting schools there to have this opportunity.
Mary
"Believe in the magic of your dreams"

One big reason for L&O to have so many broadway actors (esp as guest stars) is that its filmed on location in New York City andhas a tradition of using New York actors in parts.

Jerry was also in the play.Promises, Promises. There is a book in our local library with the play and also pictures of him starring in the main role. I believe he won a Tony for it.
Mary
"Believe in the magic of your dreams"

Nice list!
Also:
Christine Baranski
Andre De Shields
Robert LuPone
Gordon Weiss
Charlayne Woodard
and probably dozens, if not hundreds, of others (480 or so eps over 20 years, not to mention LOCI, LOSVU and LOTBJ).
"All you need to start an asylum is an empty room and the right kind of people."

Also, one of my absolute favorites, Elaine Stritch who was in Sondheim's "Company" and her own one woman show. She played a feminist defense attorney a few times. Jesse Martin sang "Razzle Dazzle" from "Chicago" at the end of last year's Tony awards as the picture of Jerry Orbach, the last one shown in their memorial tribute, was shown because Orbach had originated the role of Billy in "Chicago". Have to tip my hat to the work you put in to find them all. Walter Bobbie and Faith Prince worked together in the revival of "Guys and Dolls" that starred Nathan Lane and Peter Gallagher.

You really were bored. lol
That's really cool and maybe not that creepy.
I LOVE Musicals

I finally got bored/curious enough to get a list together of the Broadway names that I recognized. There are probably more, but I went through the lists of guests on the L&O series and put down the names I know which I'm sure neglects guests who were on in the earlier years/haven't been on Broadway as recently.
Here goes:
B.D. Wong (from the SVU series) was recently in Pacific Overtures
S. Epatha Merkerson has also been on Broadway
Some of the non-regulars and many less known (you can look them up on www.ibdb.com, the internet Broadway database):
Dick Latessa
Philip Hoffman
Tovah Feldshuh
Mary Testa
Walter Bobbie (performer and director)
Michele Pawk
Chip Zien
Beth Fowler
Malcolm Gets
Faith Prince
Leo Burmester
Fuschia Walker
John Cameron Mitchell
James Naughton
Patti LuPone
Taye Diggs
Judy Kuhn
Karen Ziemba
Donna Murphy
Roger Bart
Terrence Mann
Charlotte d'Amboise
John Cullum
Randy Graff
Nick Wyman
Norbert Leo Butz
Wilson Jermaine Heredia
Audra McDonald
Rob Bartlett
Ming Lee
Sara Ramirez
Tammy Blanchard
Christine Andreas
George Hearn
Liz Larsen
Liz McCartney
Barry Bostwick
Jane Krawkowski
Anthony Rapp
Rebecca Luker
Jeffrey Carlson
Alfred Molina
Angela Lansbury

Jesse Martin, SUCH a hottie, originated the role of Tom Collins in Rent, and will be reprising said role in the movie

There's a huge number of Broadway actors/actresses who have been guests on the Law & Order series. I can't go through the whole list now because it would be too many, but if you look through the biographies in playbills you'll see a lot have been on L&O.
I think it's probably mainly because L&O is filmed in NYC, so Broadway people have more opportunity to be on it rather than shows that film in LA.