From other people's lists:
Ani DiFranco [she's on almost everybody's list]
Betty
Uncle Bonsai
Jeffrey Krause (if you count him singing in my living room)
Into the Freylakh
Dar Williams
Elton John
The Pretenders
some others off the top of my head:
Ella Fitzgerald
Herb Alpert & the Tijuana Brass
The Count Basie Orchestra
Pete Seeger
Arlo Guthrie
Elvis Costello
Robert "Escalator of Life" Hazard (on the bill with Elvis)
U2 (just before they hit it big in the U.S.)
Bette Midler
k.d. lang
The Baltimore Symphony Orchestra & guests there like Hilary Hahn, Radu Lupu, Evgeny Kissin
El Duke-O
Alison Krauss
Mark O'Connor
Bobby McFerrin
Tito Puente
Natalie McMaster
Etta James
the line-up at that March on Washington stadium show, including k.d., Melissa Etheridge, George Michael, Chaka Khan, the Pet Shop Boys, and Tipper Gore herself
embarrassing, unsatisfying, and/or "eh":
Billy Joel
Van Halen
David Bowie
Melissa Ferrick
Flock of Seagulls (with Elvis & the Escalator guy)
White Zombie (some goofy metal band Barry got free tickets to)