Free Download: MSDN Library
Starting with the May 2006 release, MSDN Library is freely available for download from Microsoft Downloads. Previously, the Library was only available for download to MSDN subscribers. For the latest release, search Microsoft Downloads for "MSDN Library".
Update: As of October 31, 2010, the latest release available for Visual Studio 2008 is: MSDN Library for Visual Studio 2008 SP1.