Um,...this means I have to come up with 20 on my own. And even having been to Lilith Fair *twice*, I can't do that. Oh well. From what I can remember...

Indigo Girls (also independently)
Sarah McLachlan
Dixie Chicks
Shawn Colvin
Mya
Chrissie Hynde (with the Pretenders?)
Erika Badu

B52s (obviously not at Lilith Fair)

And Philip Glass - does he count? :-P