As a side effect of Team Fortress 2 becoming permanently free, Valve will make its Source SDK free as well. I wonder if people who paid to get access to mods in the past will come out of hiding and demand a refund. Wouldn't surprise me, sadly.
Says Robin Walker of Valve: "We are in the process of getting it all done. It's a bit messy because we have multiple versions of the SDK, and there's some dependencies we need to shake out. But yes, the gist of it is that we're just going to go ahead and make the Source SDK freely available."
These guys sure do treat us well.
Not Only, But Also: Source SDK To Be Free [Rock, Paper, Shotgun]