With the release of Team Fortress 2 as a Free-2-Play game, Valve Software is in the process of releasing the Source SDK for free as well. from RockPaperShotgu.com.
The Source Software Development Kit, for those of you who’ve never clicked the Tools tab in Steam, contains everything you’d need to make a mod, except for personal ambition. It has everything from the infamous Valve Hammer Editor to Face Posers and Model Viewers. At the moment, to gain access to the Source SDK you have to purchase a Source based game such as Half Life 2, or as the official website states, Team Fortress 2. Which is of course now free.
I recently downloaded TF2 when I noticed that it was free. This is a fun game and I am thoroughly enjoying it. Some mods are even available for it on different servers in the In-Game Server Browser.