GaijinPot is a great place for people who are looking for jobs, or just doing research about living and working in Japan. I recommend checking this website out if you are looking for a job in Japan.
Whether or not you will be joining our ranks of ALT or not, r/movingtojapan has a lot of discussions you can scroll and search through about how moving to Japan is like. r/JETProgramme has a lot of information for incoming JETs!
ALT Insider has a bunch of great resources and are affiliated with various websites to tell you about the English teaching world, and updates on news and happenings. It's a great site if you want to learn more about working in Japan as an ALT.