Zero-K Name:     OR   Password:   

Download and installation instructions

Download Zero-K client

Note: Windows XP not fully supported!

==and other Linux==

Create a writable folder, then execute:
sudo apt-get install mono-complete
wget http://zero-k.info/lobby/Chobby.exe
mono Chobby.exe

For alternate Linux instructions check here

Unsupported Alternatives

Weblobby (Windows and Linux)

Download here

Mac OS

While Mac is not officially supported, multiple people have had success by following the instructions at How to play on a Mac.