This is Steam. It's the game distribution system that Valve are using for Half-Life 2 and others.
You purchase the game online, and it downloads the content to you. It's similar to the iTunes Music Store, except that you don't have to "bless" the system it's installed on. Log on to Steam on a new box and it logs you off on the old one.
It's a pretty ambitious distribution system, and one that could easily fall on it's sword through the huge volume of traffic that it is handling. However there's some heavy duty server replication going on behind the scenes. As you can see, even in my own remote corner of the internet, it's being downloaded at quite a respectable speed.
Even better, the content is being served through a peering network. Internode own the servers that I'm downloading from. In return they get a banner ad. Their traffic is connected through PIPE Networks to my ISP. Result is good throughput, and downloads that don't count towards my monthly quota. I like it.