https://www.youtube.com/user/AfreecaTV for GSL
https://www.youtube.com/user/ESportsTV for SSL and ProLeague
https://www.youtube.com/user/MapControlCO for BaseTradeTV's broadcasts (they are a community stream that covers many online only events)
Anything else you could possibly ask for: https://www.reddit.com/r/SpoilerFreeSC
Live events, check http://teamliquid.net for upcoming events and announcements and stuff!