The gaming industry has been using peer to peer downloads for a long time AFAIK. Unsure if they still do, pretty sure the Battle.net client did it.
Steam never used P2P downloading but they've had deals with ISPs and communities really early and made their own "CDN" full of caching servers everywhere, so if you waited a few hours to update, it might make it to your local server in time. Now I think they're on Akamai as well.