Open Source Ajax Push Engine: APE
The core of APE is an epoll-driven HTTP server, entirely written in C, which provides patterns such as XHR long-polling (cross browser), forever frame and many more.
APE only uses web standards, no plugins are needed to be installed on the browsers & it works with all browsers.
It is possible to create various multi-user applications with APE as it can broadcast to hundreds of users with ease.
With a module support (currently with C but PHP support is on the way) the application can be extended & customized more.
Compatibility: All Major Browsers