This page gives all the basic rules available for free on the site created by Wizards of the Coast.
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!
A Wiki gathering all the information and rules from D&D5.
A web-based version of the 5th Edition SRD (System Reference Document)
A questions' directory on Dungeons & Dragons answered by designers