Comic Zeal Comic Reader The original. Organizes your library of scanned and non copy protected comics. Features seamless issue to issue reading and a slick interface. Great for your golden era collections.
Image Comics For buying reading and all your Image published comics.
Comics Aka Comixology. Features a decent selection of free comics. Same day as print releases from some publishers and the biggest selection of publishers available, including DC, Marvel, Archie, Vertigo, Valve and many more.