Most of the MSDN Library content has already been available online for free, but there is some content that is available only to MSDN subscribers. Microsoft are now giving away the content as a free monthly download – see here for more details.
Microsoft seem to be on a spree of giving away free software right now, as they also just annouced that they are giving away their virtualization software, VirtualPC. Hoping to placate customers and developers frustrated by Vista being delayed for so long perhaps?