startrekroleplay:

Memory-Alpha is the official Star Trek wiki, which is exceedingly well run and has articles for everything you can think of.

Memory-Beta is the wiki for Star Trek secondary canon (novels, comics, video games, etc.) and a good resource for finding additional information on something that isn’t covered much in canon.

TrekCore has high quality screencaps from every series, as well as audio clips, ship schematics, and more.

Star Trek Minutiae has a lot of things, including fonts, logos, scripts, a warp speed calculator, and starship size comparisons.

Ship Schematics has more images and schematics for ships than you would ever think existed.

Cygnus-X1 also has a lot of detailed schematics.

Chakoteya has episode transcripts from every series.