Bay 12 Games Forum

At its core, it's Skype meets IRC meets Mumble done right.

Completely free, no shady IP shenanigans that Skype does, lightweight, can be run in a browser or in a client (only supported for Windows and Mac, browser version runs in Chrome, Opera and Firefox), has text AND voice channels (with fairly decent audio quality) and seems to be an all-around amazing replacement for something like Skype or IRC.
And judging by their upcoming list of features, it's only going to get better and better, with stuff like bots and in-game integration akin to Mumble.

Registration is even simpler than in most other places since if you want to, you don't even need to put in a password or email (only if you want to use the browser version though, otherwise you need to verify the 'mail).

Also supports Markdown, embeds most file formats into the chatroom itself (images, videos, gifs that only play if you hover over them etc.).

The B12 'room is here.
Not sure how many of these things exist already out there, but eh, it doesn't hurt to have more ways to talk to each other, no? :v

*disappears into the abyss yet again*