Alien Swarm (Free Steam download)

Alien Swarm is an isometric third person shooter developed by Valve.  After joining a game or hosting your own you get to choose from 8 different characters and 4 different classes, each with their own unique abilites, strengths and weaknesses.  It then plops all of you down into a space station teeming with icky crawling aliens.  Each map has a different objective that you must complete while you battle and try to stay alive.  Some of the later levels get quite difficult so I suggest teaming up with others.

As you progress you will level up in rank, granting access to improved weapons and supplies, the key to victory is building a team with the right combination of these and working together (also not being caught by those damn little infester aliens).  My personal favorite: the tesla cannon, it shoots beams of jumping electricity and deals no friendly fire, aww yea.

Here is a link to the Steam website if you do not already have it.

Video Alien Swarm Gameplay.

Minecraft

for my first entry I will briefly talk about Minecraft.  If you haven't already heard about it you should definitely check this out.  Minecraft is free to play and there is also a pay to play version.  The idea is that the world is made of many different types of blocks.  These blocks can be destroyed, harvested, and used for building.  In the free version it is an open sandbox where you can create almost anything you can think of and show it off to people or play around in your little house.  Some people have even made entire cities or even replicas of things like the entire world of pokemon.  The pay to play version also has many other features like a survival multiplayer mode.

Heres a video of some people playing free version.
(EDIT: new video posted old one was taken down for some reason)