CGC (a.k.a. Certified Guaranty Company, formerly called Comics Guaranty Company) − Started in 2000, it's the oldest and largest professional 3rd party comic book grading company, located in Sarasota FL. Trained graders certify a grade for your comics and magazines based on tight standards, check for restoration on older comics, and encase them in a nice hard plastic holder (commonly called "slabbing"), for varying fees. You can also search their Census database (you have to create a free account to access it) to see how many of what issues have been graded in every grade. Their comic forums are a good resource for all kinds of knowledge, and to keep alerted to scammers and bad sellers, of which there are many, mostly on auctions. You may also want to check out this forum thread that explains various productions flaws--how they happen, what they're called etc., it's very informative: Q&A About Comic Production Flaws
CBCS (Comic Book Certification Service, LLC) − CGC's former head grader started his own grading company in the summer of 2014, so you have another reputable choice to get your comics graded and encapsulated. Located in St. Petersburg FL. Pretty much the same tight grading standards, very similar 'slab/case', same numeric scale as CGC. Has a forum too. Other than these 2 companies, I don't recommend any of the others, and you can do your own research to see why.
Comic Book Database (cbdb.com) − The most detailed comic book resource on the net! Use this free, interactive site to look up your favorite characters, artists, writers, or comic book series. The database currently comprises over 50,000 comic books and is growing daily. Many ways to search it too, so if you can't remember the exact issue number, but know the story title, or artist, writer etc., you can find the issue eventually, or simply find out all kinds of other info about the issue, like guest stars, creative talent etc.
DC Comics Database on Wikia.com − World's largest DC Comics encyclopedia; tons of info on characters and titles, news, videos, community forums, etc.
Diamond Comics Dist. − The main wholesale comic book Distributor. Keep up with what's shipping when, news, Previews order book, etc. Good resource for dealers and fans.
Gemstone Publishing − Publishers of the long running "Overstreet's Comic Book Price Guide," and other collectibles guides for toys, rings and more.
Grand Comic Book Database (comics.org) − For all you people just searching for pics of comics, this is the place to go. With several hundred thousand covers scanned, and other info about issues, this site's goal is to index all comics made! (NOT the same site as the Comic Book Database listed above.)
Marvel Comics Database on Wikia.com − Huge resource for info on Marvel Comics characters and titles, plus news, videos and forums.
Mike's Amazing World of Comics − An excellent database site, originally started for DC comics, but now includes most other major publishes. It has regular databases for covers, publishing chronology, and even to find reprints, and the Newsstand page lets you search for specific months/years to see all the comics that were published that month. Nice for buying presents from the month someone was born, or just start a collection based on particular years/months. Remember that the comic COVER date is different than the CALENDAR date the comic hit the stands, as most comics are cover dated well after the actual release date.
Sci-Fi Art Directory − A directory/links site for science fiction art, artists, books, videos and more.
Virtual Toy Chest − An online archive of toys from the '80s, '90s, and '00s. Check out dozens of toy lines and hundreds of pics!