Socket.io - Realtime application framework (Node.JS server)
Xmind - Xmind is a brainstorming and mind mapping application.
Ably - Serious realtime infrastructure
MindMeister - Create, share and collaboratively work on mind maps with MindMeister, the leading online mind mapping software. Includes apps for iPhone, iPad and Android.
Pusher - Pusher is a hosted API for quickly, easily and securely adding scalable realtime functionality via WebSockets to web and mobile apps.
FeatureMap - FeatureMap story mapping, simple and effective realtime collaboration and collective intelligence tool.