STFC Database: This is an amazing website which has all the information on every ship, hostile, system, building and research, including costs, power increases, abilities, etc. I use this site almost every day.
Spocks.club: This website is amazing. It has the best combat log analyzer, and will help you plan all ship upgrades and building upgrades, showing exactly what materials you need based on your current research if you import all the data to the website using their tool. Extremely helpful!
PVP Target: A PVP guide website, with a lot of terminology and help for PVP. Especially good if you're just getting into it and need to understand the basics and ROE, OPC, 0-Node, etc.
Gherkin's Territory Helper: This shows the territory map, when your territories are under threat and by whom. It will also help you to plan out future territory captures.
STFC Data: A German crewing website with many crews and information on how to use them (in German).
Star Trek Fleet Command Officer Tool: This spreadsheet is used to put different officers on different ships, find crews, look at away teams traits, and so much more.
The Academy: This channel has a ton of content about many different aspects of the game.