For people and teams developing mods and games with Valve's Source engine.
Gabe Newell: "We’re spending more time on tools development now than anything else."
Posted by Varsity on Apr 8th, 2011
Steamcast: MasterGir asks: ‘Will Valve ever update the source SDK, especially Hammer?’
Gabe Newell: Oh yeah, we’re spending a tremendous amount of time on tools right now. So, our current tools are...very painful, so we probably are spending more time on tools development now than anything else and when we’re ready to ship those I think everybody’s life will get a lot better. It's just way too hard to develop content right now, both for ourselves and for third parties so we’re going to make enormously easier and simplify that process a lot.
Any new tools will probably run in the engine, like the Particle Editor.