The Tor Project is the non-profit organisation that develops the software behind the Tor network and the Tor Browser. This is their documentation.
BridgeDB
BridgeDB is The Tor Project's database of Tor bridges. They're very useful if you live in a country that censors the internet and blocks Tor entrances.
EFF's SSD Guide
The Electronic Frontier Foundation has made a website dedicated to information about protecting yourself from any kind of surveillance.