To download Sitecore and associated official modules, you must be a Sitecore certified developer or partner. To become a certified developer, you must attend developer training and pass the exam.
The Sitecore Experience Platform and all official associated modules can be downloaded from the Sitecore Developer Portal. This site hosts all downloads and updates after Sitecore 8. This is also where you will find the installation guide and upgrade instructions for all versions of Sitecore including and after 8.
All versions of Sitecore and associated official modules can be downloaded from the Sitecore Developer Network. Any updates to these versions (e.g. Sitecore 7.5 Update-2) will also be posted here.
The marketplace offers free modules, some of which are supported by Sitecore. Some modules are either tested by or supported by Sitecore, but many are built and maintained by the commnity.
There is a formal Sitecore GitHub and an informal Sitecore GitHub. Community docs are hosted on the informal version.