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. Anyone can now check the certification numbers and see scans without joining at https://www.cgccomics.com/certlookup/ . 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
Comic Vine − Claims to be "the largest comic book wiki in the universe". Covers TV and Movies too. Multiple ways to search and browse. Has busy forums too.
DC Comics Database − 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.
GPAnalysis.com − GPA compiles and reports auction and some dealer sales of CGC graded comics. It's a paid-for subscription service, but if you're looking to buy high dollar graded comics, or sell yours, it's worth the price to check what they're selling for. Be aware that prices can vary widely for rarer comics that there are few sales of, or that have big time breaks between sales, so it's still a guessing game sometimes. And a fair amount of auction sales don't get completed or are fake, shilled, canceled and relisted, but are 'counted' as sales, so that can skew prices, even though they try to track and account for that. But this is the oldest and main CGC comics price 'guide' service around, and they give you multiple types of reports and charts.
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 − 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.
Toyzine.com − Your global toy and collectibles connection. Huge selection of vintage toys and collectibles pictured for sale or auction, toy and collectible auction coverage, wanted classifieds, informative and entertaining e-magazine, and much more.