Following the free to play release of TF2, Source SDK has been accidentally released free, and will remain that way.
This news comes a little late, but for those unaware due to the recent release of a free Team Fortress 2, the Source SDK will soon become free to use in its entirety, for everyone (confirmed by Valve's Robin Walker in this RockPaperShotgun article). The free SDK was first uncovered in Moddb's forums on the 28th of June by an enterprising young man going by the name of Riley, who just happens to be the playtest leader of esteemed Source mods Zombie Panic: Source and No More Room In Hell.
This means that all Source mods, made for Valve games (we list around 1,500 for HL2 alone) may eventually be free to play, without requiring you to purchase a Valve game (thou who hasn't purchased a valve game on this site?)
When RockPaperShotgun emailed Walker for further confirmation, and he went on to explain: