Here's a handy resource guide that pulls together everything you will want to know right now about Apple's new OS X, version 10.11, called El Capitan: Apple's official info, the beta program, and technical introductions.
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1. Apple's primary introduction and overview page for El Capitan with a list of features.
2. The El Capitan introduction page for OS X developers.
3. Information on how to sign up for the Apple Beta Software Program for OS X El Capitan (and iOS 9).
4. System Requirements for El Capitan at TekRevue.
5. Apple's official press release on El Capitan. Note that Apple doesn't make much of the version "10.11" nomenclature except in technically focused discussions. For casual reference, it's "OS X El Capitan."
6. Features of El Capitan that Apple didn't show in the WWDC keynote at 9to5 Mac.
7. A high level overview of Metal for El Capitan at iMore.
8. A video introduction to El Capitan by MacRumors.
9. The wikipedia page for OS X with notes on development history and release dates.
10. Which Macs Support OS X El Capitan’s Metal?
Apple has announced a public beta program starting in July and full release "in the fall" that will be free. While that, technically, gives Apple until December 21, 2015, I believe it's likely that Apple will shoot for an October release, as it did with OS X Mavericks and Yosemite. If you find any other great links to add to this list, please include them in the comments below.