If you havnt heard already on the 3rd of October valve added 5 leading Source MOD games ready to be installed to your computer. These games are entirely free and hopefully will stay free.

What are source MODS?
They are modified games, built from valves leading engine technology aka source engine

What you will need to play this?
You will need any half life 2 game, such as Counterstrike Source, Team Fortress 2, Day of Defeat source etc.. If you do not have any of those games valve is offering Half-Life 2: Episode One for $4.99, this is only for a limited time

What are the games?
The game are Age of Chivalry, D.I.P.R.I.P., Insurgency, Synergy, and Zombie Panic.

How to download the games?
Right click on the steam icon in taskbar (bottom right hand corner of your computer), click on games, scroll down to the bottom and you will see the Not Installed games, then download the ones you want

Which games you MUST try?
Have only played 2 of the games, which is Synergy and Insurgency, I will explain both below:

Synergy:
Syngery is a multiplayer game, based from half life 2, its basically a singleplayer scenario made into a multplayer game, you and your team go around killing aliens, soldiers, and all these funny creatures, This game is highly recommended

Insurgency:
Insurgency is an online combat game, very interesting and also highly recommended

As for the other games you can check out for yourself