I’ll start a living list of educational online resources to enrich your teaching. I’ll eventually make it into a more database-y look when I have time.
My Free Bingo Cards
Jeopardy Labs
Kahoot

https://kahoot.com
Create an interactive quiz game that invites users to use their own devices to participate. Works via remote meetings too
Slido

https://slido.com
This one is more about meeting interactivity and polls, but it is interactive, has an app and a web interface, and can be embedded into a PowerPoint, delivered remotely
Poll Everywhere

https://polleverywhere.com
Poll Everywhere is some good solid interactive poll software. Free, similar to Slido but perhaps a bit better. Slightly different implementation but very easy to use
Mentimeter

https://www.mentimeter.com
Mentimeter is a combination slideshow tool / with integrated feedback/polling. Good for audience engagement and live information gathering to emphasise points.
Certifying attendance at sessions
https://www.certifyem.com
Certify’em has just what you need to generate and issue certificates online: customizable templates, email delivery, detailed records, and QR codes for verification.

