Dear Elliotte,
I'm afraid it is not that simple.
Out of my head and maybe with some mistakes ...
a) Near & Far designer from Open Text
uses James Clark SP
b) XML Spy from Altova
uses it's own parser
c) XML Authority from TIBCO
Crimson ?
XML editors
a) XML Spy from Altova
own parser
b) XML Instance from TIBCO
Crimson ?
c) XML Pro from Vervet Logic
IBM XML4J
d) XMetal from Softquad
own parser
e) Epic from ArborText
parser based upon mark-it
Parsers
a) Xerces-C
Works.