You can try the full game first for free across this weekend, see details below:
The "Free play" weekend (as found by Lonfar) is confirmed. All you have to do is:
Cut'n'paste this link into the browser you are using steam://install/25700 and it should start up your steam client, if not then do this...
1) Load up your Steam client
2) Goto the "My Games" tab
3) Scroll down to the "Not installed" section
4) Right click "Madballs in... Babo Invasion"
5) Click "Install game", it's only 320MB
6) Wait for it to finish downloading and you can try the full game for free until the weekend is over.
The S.W.A.T. Combo pack (DLC for the game) is also 75% off @ £2.24: http://store.steampowered.com/sub/2752/
The spiritual successor to the massively popular freeware top-down shooter BaboViolent2, Madballs in Babo:Invasion is an arena based 3D shooter offering players a satisfying and immersive experience in both single-player and multiplayer, including 'Invasion Mode' where players are able to design their own combat maps almost instantaneously by placing a combination of map tiles to create a never-before-seen map before gameplay starts. Truly, no two games are ever the same.
Featuring the recently re-released Madballs characters, Oculus Orbus and Horn Head, from the popular 1980s toys best remembered for their outrageously bad looks and graphically colourful names, Madballs in...Babo: Invasion is a feature laden 'casually hardcore' action shooter.