Riot.im

Riot.im is an IM app to share messages, pictures, videos, and other files of any size with users or groups. You can also hold one-to-one videochats via WebRTC protocol.

To use Riot.im you do need to be registered, which requires a valid email account. You can optionally add your phone number to make it easier to add contacts, but you don't have to. Plus you can invite other users by email.

Besides directly exchanging messages with other users, you can check out any of Riot.im's hundreds of chatrooms, each of which can host more than a thousand users at once. And your chat history will sync across all the devices you use, as well.

Riot.im is a chat app that's actually much more than just a chat app. With it you can exchange files and docs of all kinds, see what's trending in one of the many chatrooms, and do much more.