Some conferences may post lectures online. For example, the Banff International Research Station (https://www.birs.ca/) hosts a 5 day workshop nearly every week on a contemporary topic with leading researchers, and you can watch live on their website. If you missed one you wanted to see, they also maintain a video archive for many of their talks.

(edited to include suggestions from Geoff Robinson and Yemon Choi)

There are also research lectures available from 

 - The Mathematical Sciences Research Institute (https://www.msri.org/web/msri/online-videos),
 - The Newton Institute (http://www.newton.ac.uk/webseminars),
 - The Fields Institute (http://www.fields.utoronto.ca/video-archive),
 - L'Institut Henri Poincaré (https://www.youtube.com/user/PoincareInstitute/videos), which contains the two latest Bourbaki seminars, as well as lectures from their thematic programs.

The first three can also be watched live.