- Warnerbros: Inception Official Movie Site
The official site is a bit like html coding gone wild.
- Inception: The Cobol Job
"The Cobol Job" is an official prequel comic to the movie (read summary), and can be downloaded for free in .pdf format. It details the extraction job for Cobol Engineering that led Cobb and Arthur to Saito. The PDF can also be found here and here.
- Inception Shooting Script
Christopher Nolan's shooting script for the movie can be read here or printed (in .pdf format).
- Wikipedia, the free encyclopedia: Inception
has a lot of information about the movie's plot, the production, and the release, but lacks for instance character portraits.
- Inception Wiki
is aimed towards being a information resource and compendium of knowledge on the movie. Like any Wiki it can be edited and added to by all visitors. However, its articles on the characters, among others Arthur and Eames, are still very basic and incomplete in regard to, say, secondary material from interviews.
- PASIV Instruction Manual
johanirae @ livejournal has posted graphics from www.pasivdevice.org that detail how the "Portable Automated Somnacin IntraVenous Device" works.
- Inception Infographic by dehahs on deviantART
is perhaps the most famous non-canonical graphic on the internet; it depicts the levels, kicks, and timeline of Inception, clarifying things quite a bit.
- An Illustrated Guide To The 5 Levels Of Inception
by Matt Sinopoli And Josh Tyler also helps in understanding the levels of dreamscape and their implications in the movie.
- Dream A Little Bigger, Darling - the Inception Fic Writer's Guide to Firearms , , , 
by chn_breathmint @ livejournal details all firearm-related information (with lots of pictures!) an Inception fanfic author could ask for: Gun Safety Rules, Anatomy Of A Gun, How Guns and Silencers Work, Ammunition, Wound Ballistics, Types of Guns, Holsters, Accessories, Correct Stance, Legality. Awesome.
- You're A Smart Man, Mr. Eames
This meta by bookshop @ livejournal is a passionate defense of Eames' character and a refutal of the partial trend in fandom to turn him into a cartoonish distortion of himself. No, Eames is certainly not stupid. So there.
- How to Dress Your Man/Character: An Informal Tutorial Heavy on the PicSpam
hackthis @ livejournal has done tons of research and produced a meta that's suited to be immensely helpful in professionally (and sexily) dressing a man in fanfic.