Valve have today announced that Steam - which already takes care of most other in-game needs for the PC market - will now be offering in-game downloadable content procurement as well.
What does this mean? You'll be able to get the DLC from the same program you're launching and managing the main game from, instead of fishing around somewhere else for it. Just fire up the game, hit shift-tab and you can shop to your heart's content.
Convenient? Yes. Just don't be surprised if that convenience comes at a cost when you find in six months that some of the map packs PC gamers used to get for free suddenly are not free.
The first game to take advantage of in-game DLC will be The Maw, but since the feature can be accessed from any game - old or new, purchased from Steam or from retail - it certainly won't be the last.