Assault Cube is opensource fps game, based on cube-engine. Polygame is currently running version 220.127.116.11. Currently there is only instagib 'länkkäri' mode dedicated server, because this mode has been most popular (at Polygame events) so far. There will be server with maprot with more maps and different game modes available; date not specified.
Server : ac.polygame.fi (volta.polygame.fi) Version : 18.104.22.168
Connecting : either by using Polygame AC Utility tool (instructions published later) or by menu(esc) -> multiplayer -> custom connect. Port is default so leave it empty.
Server password: polycube
Windows: Load correct install package from http://sourceforge.net/projects/actiongame/, make sure it's of correct version(22.214.171.124) and install
Linux: Try using your systems package manager for installing. Some distributions have assault cube there.
If not, then:
load correct install package from http://sourceforge.net/projects/actiongame/, extract files, and run make in folder you extracted the files.