HyperLobby is a client application to join an online gaming chat room. It was originally intended to support some online games as well as online flight simulation. The client is very lightweight and it runs on almost any Windows version (it's written in VB5), and the server component features a complete core architecture build in C. Because of this, it's very lightweight and efficient, supporting a lot of load, and it can run over Linux seamlessly.
One interesting option is the Pager: it just sends a private message to some user, powered by a message queue for those who are currently offline.
But HyperLobby is not just a communication tool, it comes bundled with an interesting set of features. For example, when you enter a specific game chat room, you can call the game executable to enter the game too.
Please note that if you are behind a firewall or something similar, you have to open the TCP port 1698 (main communication) and the ICMP echo (for lag calculation).
HyperLobby is also known as HL.