Most comic book companies do not accept unsolicited submissions via mail or email. Here are a few who do:
Antarctic Press – Small indie publisher formed in 1984 that focuses primarily on “American Manga” style books with titles such as Robotech, Gold Digger and Ninja High School.
Dark Horse Comics – Major publisher of a lot of great original and licensed titles like Star Wars, Conan, Akira, Usagi Yojimbo, Aliens, The Mask, Sin City, 300 and Hellboy.
Image Comics – One of the largest creator owned comic book publishers with such well known titles as Spawn, The Savage Dragon, Gen13, Witchblade and The Walking Dead. Note: Image only accepts completed proposal submissions. (i.e. series synopsis and sample pages of artwork)