Since the medium of comics arguably originated in the USA, it should come as no surprise that a whole lot of comics are American. Also included here are Canadian comics, as they are for the most part pretty similar to their southern counterparts due to the large amount of intermingling between the two cultures, businesses and talents, and often see distribution in both regions regardless of origin.
Companies prolific enough to have their own pages:
Aardvark-Vanaheim
Abrams ComicArts
ACC Studios
Ait / Planet Lar
Antarctic Press
Arcana Studio
Archaia
Aspen MLT
Avatar Press
Black Bull
Broadsword Comics
Broadway Books
Caliber
Charlton
Comico
Comics Interview
Continuity Comics
Department Of the Army
Devil's Due
Disney
Dork Storm Press
Dragon
Dynamite
EC Comics
Eclipse Comics
Eros Comix
Event Comics
Evil Twin Comics
Fantagraphics
First Comics
First Second
Fishwrap
Gold Key
Hannagram
Harper Perennial
Harper Teen
Heroic Publications
IDW
Innovation
Kenzer
King Features
LEGO
Mariner Books
Minx
Mirage Publishing
National Hockey League
NEC
Nedor Comics
Nuage Editions
Octopus
Oni Press
Pantheon Books
Paws
Platinum Studios
Radical Comics
Red 5
Rip-Off Press
Roaring Book Press
Sega
Scholastic
Shanda Fantasy Arts
Simon & Schuster
Slave Labor Graphics
Studio Foglio
Tekno Comix
Top Shelf
Topps
Tower Comics
Tundra Comics
UDON
United Feature Syndicate
Valiant
Villard
Viper Comics
Wildstorm Productions
Zenescope
Independently published