If you are a MSDN subscriber, you can download your VS 2008 copy from the MSDN subscription site - http://msdn2.microsoft.com/subscriptions
If you are a non-MSDN subscriber, you can download a 90-day free trial edition of Visual Studio 2008 Team Suite here -
90-Day Trial Downloads
Visual Studio 2008 Professional Edition (smaller in size)
Visual Studio Team System 2008 Team Suite
Visual Studio Team System 2008 Team Foundation Server
Visual Studio Team System 2008 Test Load Agent
If you want to use the free Visual Studio 2008 Express editions (which are much smaller and totally free), you can download them here.
If you want to just install the .NET Framework 3.5 runtime, you can download it here.
If you are looking for Virtual Machine (VPC) with Visual Studio 2008 Installed for You then check here -
Visual Studio Team System 2008 Team Suite with Visual Studio Team System 2008 Team Foundation Server
Visual Studio Team System 2008 Team Foundation Server with Team Explorer
To use these virtual machines, you must have either Virtual PC 2007 or Virtual Server 2005 R2 SP1.
If you want free MSDN library it is avaialble for download here - http://go.microsoft.com/?linkid=7947400
To help you jump start with new features with VS 2008 you can download free Visual Studio ebook here - http://go.microsoft.com/?linkid=8010142
Keep watching this blog if you are interested in exploring new features on VS 2008 apart from this ebook. Happy exploring! Cheers - Dipesh