One positive effect of the various lock-downs in place around the world is the sudden emergence of massive numbers of online seminars and talks. The site Math Seminars, developed by a team at MIT and currently in beta, holds a list of upcoming one-off talks and seminar series.
At the time of writing, there are 182 talks and seminars listed in April 2020 alone. These can be filtered by topic and language and display using your local timezone. Current listings include talks and seminars in English, Spanish and Russian which can be filtered across 33 topics in mathematics, education and history.
As well as the ability to browse, filter and search the listings, the website allows you to register to receive online links, export talk and seminar details to your calendar and add your own listings.