It's a chat platform and you can sign up for free. One can upload files and images, and it can tie in to other platforms (though I think these are all workplace oriented). However given how widely used it's becoming by business I have my doubts that it will continue to be so open to use.

Actually the thing that caught my attention was IRC use in 2004. If someone had quizzed me, I would have assumed its peak use was at least 3 years earlier!