With the launch of Flash 10 there is a P2P technology Stratus built right into flash. When it was launched I was not really interested in it as it takes time to deploy the latest flash runtime. But in less than an year Flash 10 has been deployed on more than 90% of the systems worldwide (According to these stats on adobe.com).
I personally don't like flash but flash is not just about animations and videos. Its a lot more than that. And these things are what will keep flash alive even after the onslaught of "HTML5". The problem is HTML5 is made out to be this magical solution that will rid the world of Flash. Unfortunately with people like Steve Jobs promoting HTML5 as the solution to all Flash problems most people blindly start to believe it.
The reason Flash really works is that the deployment is so large and most people have it installed even if they don't know about it. iPhone got away without using flash as they had partnered with Google when they first launched the iPhone. They got an application built for it that would stream YouTube content so most people really did not miss flash on one of the biggest sites which uses flash. With the strong momentum that iPhone had flash got sidelined and the only place where we now think of flash is for Videos or Animations.
With Stratus you can do things that till now were not possible without setting up a huge server farm. List of webapps using Flash based P2P KKloud Cloud , PipeBytes, Files Over Miles and JetBytes. Even Chatroulette uses Stratus to connect users together. Since there is no server in between the cost of hosting video and voice chat is almost minimal.
There might be more applications which are using it and I will try and keep this post updated with new services which use it when they are launched.