AKA MSDN. MicrosoftCorporation's subscription-based service for developers.

MicroSoftDeveloperNetwork includes two main features:

1. Technical support, thorough documentation, tutorial and demo materials form various MicrosoftCorporation WebSites shipped on CD or DVD so developers can install and consult it locally. Sufficiently highly-paying customers can even install it on a LAN for all developers within an organisation.
2. Access to support forums in which developers are guaranteed an answer. Usefulness and meaningfulness of the answers are not guaranteed.

The MicroSoftDeveloperNetwork library is often bundled with other products, typically various VisualStudio packages.

See: http://msdn.microsoft.com/