The Royal Opera House has regular performances online.
The Met Opera is streaming titles from its "Live in HD" series through its website. A different opera every day available for 20 hours.
The Ring Cycle (Opera North) will occupy you for hours and hours.
The Mariinsky Theatre in St Petersberg has just started a series of on-line broadcasts of opera, ballet and concerts. Click on Mariinsky.TV — live webcast. Also from Russia the Bolshoi is presenting a series of their Cinema Broadcasts for free: https://youtube.com/user/bolshoi