DnDBeyond
This page gives all the basic rules available for free on the site created by Wizards of the Coast.
Rules
Here you can find the links to different sites giving Dungeon and Dragons' rules. If you know other links, don't hesitate to suggest them on the home page!
The websites
DnDBeyond
Dungeons & Dragons 5th Edition wiki
A Wiki gathering all the information and rules from D&D5.
5th Edition SRD
A web-based version of the 5th Edition SRD (System Reference Document)
Sage Advice D&D
A questions' directory on Dungeons & Dragons answered by designers