It isn't an add-on, a mod or a new way of playing the world's most popular online shooter. Counter-Strike: Source is the same game that's known, loved and often responsible for gamers sitting motionless for days on end. But it's now been ported to Valve's new 3D engine, the Source engine, giving us our first glimpse of the technology that will be powering Half-Life 2.

The new version is being distributed online via Valve's Steam distribution software. The Steam system (www.steampowered.com) is worth talking about in its own right, as it could mark the beginning of a revolution in software delivery. Steam isn't just a download mechanism: it integrates DRM (digital rights management) to handle encrypted pre-delivery of unreleased software, allowing it to be played only when it's been officially published; automatic software updates so that you no longer need to worry about patches (see somewhat above); and software management, giving you a handy list of your downloaded titles and, of course, notification of newly available games. Valve is actively promoting Steam as a generalised, licensable delivery system too. If its first baby steps into the world are successful, we're sure to see it being put to use by third parties before long.

It's the same game as before, but firing up Counter-Strike: Source for the first time is still likely to induce an 'oooh!' in most veterans. It looks pretty, the volumetric fogging effects in particular adding an extra dimension to maps like Dust. That said, most of the new capabilities of the Source engine don't have an immediately obvious effect. But immediately obvious effects aren't what Valve is about. Subtlety, the effect that's tastefully placed to enhance the experience, is what makes its games so compelling. The Source engine's main claim to fame is an enormously improved in-game physics engine with ragdoll modelling, plus an extremely sophisticated character body and face animation system. The latter isn't much use in Counter-Strike since you rarely have time to stop for a chat, but the ragdoll physics certainly enhance the atmosphere.

But it's the other improvements, mainly DirectX 9 shader-based rendering, that provide the most enhancement. Indoor areas with stone floors show gorgeous specular reflections from sunlight streaming in through doorways; move into darkened areas with high windows and shafts of light filter through, lighting dust motes floating in the air. But the best gameplay-enhancing example comes when flashbang grenades detonate in your vicinity. The sudden white-out is now accompanied by blurred and grainy vision, plus a piercing whistle in your ears that actually does feel as if you've been stunned. All too often, you'll come round just as the enemy is placing their gun to your head.
It's ironic, though, that the best demonstration of the Source engine comes not in-game, but though the menu-accessed video stress test, used for assessing the level of graphics detail. Those who've been following the development of Half-Life 2 will recognise the routine from videos floating around the Web from events such as E3.

It's a demonstration of the Source engine proper, featuring both refraction and fresnel water effects, plus fire and in-game portal-style video camera views. It's mouth-watering in its capabilities and provides nothing but frustration for those of us who are longing for Half-Life 2 with a fervour usually reserved for bedroom-related activities.

You could accuse Valve of being a tiny bit disingenuous by releasing its first Source-engined offering attached to a game that can't make full use of its features. There's no scope for the AI system; for appreciation of the skeletal and muscle-based facial animation architecture; for deformable terrain features; for ancillary environmental effects like rain or fire; for vehicles: the list goes on. But, you can't knock a software company for keeping its existing portfolio up-to-date with the latest technology, and given that Counter-Strike: Source is a 'free' game once you've paid to preload Half-Life 2, we can't complain. To be sure, it's not much more than a distraction while we're waiting for the main event, but it's a distraction worth its salt.