Valve puts popular mods on to Steam for free

"As a part of our continuing efforts to support the MOD community, we will begin hosting selected MODs directly on Steam starting next week.

The first five MODs to ship on steam will be Age of Chivalry, D.I.P.R.I.P., Insurgency, Synergy, and Zombie Panic.

As always, owners of any Source game will be able to download and play all of these MODs for free."

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