Thank you to Winged Nazgul for pointing me to the fact that Steam is doing a Madballs special this weekend with free play for the weekend and 75% off the game itself. Madballs is a fast paced third person shooter with single player and multiplayer elements. I have dabbled a bit in both and it seems like a lot of fun. The plot is completely incomprehensible but all you really need to know is that you play a small ugly ball who rolls around a lot and shoots stuff. I guess the best description is that it is kind of mad! If you have any interest at all in an anarchic shooter then now is the time to indulge because the Steam sale has ensured that there are plenty of people around to play multiplayer with.
If you buy the base game then most of the characters and weapons are locked but you unlock them as you play through either single player or multiplayer campaigns. A nice touch is that impatient types can spend a few pennies (literally) more for the anarchy unlock pack which unlocks everything from the get go.
Warning: Do not buy the anarchy pack if you enjoy unlocking stuff as you work your way through the game. Once your buy the anarchy pack everything is unlocked and you cannot go back according to this post on the forums. I played a bit more multiplayer and it is a total blast. Well recommended.