It's official - from next week many Source mods will be available directly from Steam as a free download, making it much easier for you to download and install, as well as to keep up with the latest updates.

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.

Once installed, these MODs will appear in your "My Games" list and will receive automatic updates just like other games on Steam. Also, these MODs now take advantage of Steamworks, which provides stat tracking and tighter integration with the Steam community.

We're excited to see MOD developers get wider recognition for the hard work they have done, and we hope to support more MOD teams in the future. Watch for the official release next week!

Source

Downloadable MODs are -

• Age of Chivalry
• D.I.P.R.I.P.
  
• Insurgency
• Synergy 
• Zombie Panic

and many many more comming soon. List.