It was used in the WoW downloader pre-Battle.NET, they've moved off that and switched to CDNs (mostly Akamai IIRC) & dynamic patching, i.e.: Battle.NET computes differences and gets a diff.

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.