The other Marx sites that I know of are (in no particular order):
Frank Bland's Why A Duck?
Stefan Timphus' The Marx Brothers
Rick C's The Groucho Zone
J. Bradley Main's Groucho Marx Slept Here!
La página Web de los Hermanos Marx (in Spanish)
Rolf Cleis's Die Marx Brothers (in German)
Kim Bruno's Groucho Marx
J. C. Kaelin's The Marx Brothers - Duck Soup Wavs
Laurent Aupetit's Marx Brothers quiz (in French)
FatJack's The Marx Brothers
Christoph Windpassinger's Meine Marx Brothers Zitate
Bob Black's Theses on Groucho Marxism
"Los Hermanos Marx - La página de Miguel" (in Spanish)
Marx Brothers in Print