It’s always frustrating to read a list of the best Steam mods, as most top lists tend to be composed primarily of unreleased mods such as Black Mesa Source and Mario Kart Source. Â Don’t get me wrong, those look awesome – but it’s been years since they were first announced and they remain frustratingly unreleased.
I have put together a ranked list of what I consider to be the best free mods currently out – no waiting, you can play these free right now. Â Feel free to comment if I missed a good one, or disagree with their rankings. Each of these mods requires that you have Steam installed in order to play (some also require that you own a Source title – list of source titles).
Alien Swarm
Alien Swarm is one of the best multi-player titles to come out this year.  It sports a unique top-down interface (it’s possibly to play it FPS if you prefer). One can easily put 40 hours into the game playing through each of the different classes, achievements, and difficulty levels.  What really set this mod apart is that it was released free of charge.  One does not even need to own another Source game (unlike most of the other mods in this list).  This game is an easy choice for the top spot. It also has by far, the largest number of active gamers at any one time of all the mods.
Zombie Panic!
Zombie Panic! is an awesome amazing astounding mod that allows a large group of players to engage in a large scale multi player team battle. Â Every match at least one player spawns as a primary Zombie, and the rest as a group of scattered survivors. Â The survivors must do everything they can to barricade themselves and work together to hold off the ever growing Zombie swarm. Â Each survivor that dies or is bitten respawns as a Zombie. The developers have announced their intentions to release a sequel call Contagion. Â The game is hugely popular, and has several hundred players playing it at any given time.
Insurgency
Insurgency is an extremely realistic CS-style online FPS that pits insurgents against the US military. Â Strong squad tactics, and one hit kills may leave those used to Counter Strike frustrated, but after playing a few rounds the game really starts to draw the player into it’s realistic battle environment. The game has a fairly strong multiplayer community, with a steady group of populated servers always available.
Pirates, Vikings, and Knights II
This game is ridiculous.  It’s also ridiculously fun.  You choose from three  different player teams to be either a Pirate, a Knight, or – yes you guessed it, a Viking.  You’re probably wondering if that screenshot really shows a Pirate flying through the air with the exploded remains of his Parrot floating over a red bearded Viking.  The answer is yes, and so yes, you should go download this multiplayer deathmatch and enjoy.  It’s rough, but it’s fun and there’s usually almost as many playing this as Insurgency (This is the mod everyone goes to when they get frustrated by Insurgency’s difficulty.
