EDDB.io
The one stop for all information on systems, planetary bodies, stations, commodities and whatnot.
Also invaluable for finding great trade routes.
https://eddb.io/

EDSM.net
Maintains your CMDRs profile, shows you your statistics and has a huge amount of information on systems, planetary bodies and whatnot.
https://edsm.net/

INARA.cz
Same as above mentioned EDSM, but with added information about ships, modules, engineers, etc..
https://inara.cz/

Coriolis.io
Want to test modules but do not want to buy every single one to do some testing? Coriolis has you covered.
With this ship builder, you can play around with every ship and all modules. Free of charge.
https://coriolis.io/

EDCodex
You will not find a bigger collection of tools and documentation on Elite: Dangerous than right here. Bring coffee, going over this is going to take a while …
http://edcodex.info/

EDDiscovery
Out of game (and this world!) tool that reads the game logs and keeps track of all your doings. It tells you where you are, what you have done, how much that planet you just scanned is worth, and much much more!
https://github.com/EDDiscovery/EDDiscovery

Dr. Kaii – EDProfiler
This tool is the end-it-all tool that will bring the fun of manually editing all those ini and config files to a halt. Anything you want to tweak, in one neatly setup interface.
http://www.drkaii.com/tools/edprofiler/

NO2O
Orange. Everything is SO orange. But it does not have to be that way. Enter the No to Orange community. A handy collection of colour schemes for your ship HUD. Find one you like, copy the data and paste it into the above mentioned EDProfiler. Blue is the new orange!
https://forums.frontier.co.uk/showthread.php/259311-NO2O!-The-Definitive-List-of-1-7-2-2-Compatible-HUD-Colour-Color-Configs-(please-add-yours!)

EDHUDEditor
Want to tweak those HUD colours even more? Create your own!
http://arkku.com/elite/hud_editor/

The New Commanders welcome pack
https://imgur.com/a/i7KFV

One of the better YouTubers who’s been posting how-tow’s for ever has now joined forces with a few guys, which has lead to an immense collection of tutorials, hints and tips:
https://www.edtutorials.com/