Stateless by design
Run as many instances as you like behind a load balancer. The hub host is encoded into the session ID, so any instance can serve any request.
Ggr
A lightweight active load balancer used to create scalable and highly-available Selenium clusters.
Quota per user
Each user gets an XML quota file describing the browsers, versions, regions and hosts available to them.
Proxies everything
Video, logs, downloaded files, VNC traffic and developer tools are all proxied through Ggr using the session ID your test already has.
Works with external services
Point a quota entry at Saucelabs, BrowserStack or TestingBot and Ggr routes to them with the credentials from the quota file.