Doctor Breen told you not to do it, but you didn't listen. You had to trigger a cataclysm, had to take down the Citadel and the Combine.

You didn't think about the consequences - the world faded to grey, the G-Man left the stage, the credits rolled. The lights went up and you stared at the screen.

But Valve will keep working on Half-Life 2. In the next six months they're going to extend it and drive their technology to new heights. This fun will not be available in the shops. Instead, Valve will release it via Steam, their online distribution network.

In traditional game development, much is wasted. Programmers make tools for level designers, and level designers work at exploring the possibilities of the technology. Then, just as they understand their tools, all the hard work is thrown away in preparation for the next project.

Valve isn't going to do that. As soon as Half-Life 2 was finished, it began working on the first follow-up, tentatively called Aftermath.

"It deals with the events and issues set in motion during Half-Life 2," says Valve's in-house writer Marc Laidlaw. "You've done critical damage to the Citadel. The whole place is going to go up, taking out City 17 and what's in its immediate radius."

So how do you go about refreshing the elements of Half-Life? "We have a concept called design economy," explains designer Robin Harper.

"Simply: how economically do we use each element? How well is an object connected to all the others? The heart of game design is making those connections, and making them more interesting."

His best example involves the gravity gun. "We think of it not as 'this gun messes with the gravity', but 'this gun is a metaphor for interaction'. Thinking up new ways of making it interact in more interesting ways is the path we've wandered down.

"The reason we're able to do this," says Marc, "is because of Steam... If we were doing this without it we'd have to put it in a box and start figuring out shelf space over a year beforehand. You'd see it six years from now."

If it works, the next step for Valve will be real creative independence, free of publishers and retail hassles. It's just going to make games. That's the future.