I don’t want to use Skype because it’s proprietary.
I’m looking for an alternative that
- is Free/Libre (FLOSS),
- is available cross-platform * (at least: GNU/Linux, Mac OS, and Microsoft Windows),
- offers audio chat with another user, and
- offers video chat with another user.
Not required: text chat, multi-user chat.
(* If there is a protocol that works with different clients, such a client doesn’t have to be available cross-platform, if there is at least one client for each OS that fullfils the other requirements.)