Ballroomdancers.com is an online resource for the ballroom dance and dancesport enthusiast with images, movies, syllabi, variations, and several glossaries.
Ballroom Guide is a site made by an Open-level couple from the collegiate competition circuit. They offer a bunch of instructional videos, competition locators, and other resources.
Dance Forums is a site that a lot of our members have used to get feedback on ballroom questions.
Daniel Libatique is a DJ for the collegiate ballroom circuit (and our own DCDI DJ). He's made playlists on Spotify for every single ballroom dance. They'll help a lot with finding music when you're practicing on your own.