Microsoft's Technical Information Network.
It consists of both Microsoft's website TechNet Online and a subscription plan which sends you monthly CD-ROMs or DVDs, both intended to help with the deploying of and support of Microsoft products.
The subscription plan includes Service Packs, Resource Kits, the vastly useful Technical Information database (including the Knowledge Base), Training Software and Materials, updated drivers and patches.
After a few months, you end up with an awful lot of pretty coasters. You can get almost all of this online now, you're paying for the convenience of having it on physical media.
www.microsoft.com/technet/