Valve has introduced the ability for developers and publishers to offer downloadable content through Steam, just as Microsoft and Sony do with their console platforms.
This means that if you own a game that lives on Steam - whether you bought it direct, at retail or elsewhere online - you can now use Valve's platform as a one-stop shop for add-ons, rather than scouring the internet for them.
It also means that publishers are now in a position to charge for downloadable content through Steam. The first example of this is The Maw, for which two new levels are available a GBP 1.10 apiece.
Valve has a well-known aversion to premium DLC, arguing that if you buy the game you should get all the subsequent content as part of that outlay.
However, the company's publishing empire is a discrete but connected entity, and doesn't tend to discriminate against clients with other ideas about how to do things.
In other words, we can expect plenty of premium DLC to follow for other PC titles, but - a dramatic ideological shift notwithstanding - free DLC from Valve.
It's also worth noting that Steam isn't the first PC platform to do this. Microsoft's Games for Windows Live Marketplace already offers premium DLC, including the Operation: Anchorage expansion for Fallout 3.