These are the ones that I find myself using often:

1. The Elite Dangerous Forum
https://forums.frontier.co.uk/forumdisplay.php/29-Elite-Dangerous

2. The Coriolis Shipyard - Very handy for checking out a ship or loadout before you buy
https://coriolis.io/

3. ED INARA - Comprehensive Database of Everything, plus screenshots and fun RP posts
http://inara.cz/

4. ED Utilities - Search Tool for finding important/cool stuff near your current location
http://www.hozbase.co.uk/elitedangerous

5. Quick n' Dirty Guide to Guardians Tech https://www.reddit.com/r/EliteDangerous/comments/8uxkma/simple_guardian_guide_weapons_and_blueprints/

6. Another Guardian Site Link
https://forums.frontier.co.uk/threads/catalog-of-guardian-structures-and-ruins.589897/

Some additional resources that other users have recommended:

1. Elite: Dangerous Database (recommended by CatGrass)
https://eddb.io/

2. Elite Dangerous Neutron Router (recommended by Nexus of Concordia)
https://www.spansh.co.uk/
Havent used this one personally, but it looks like it could be an invaluable resources for long range exploration