So big news today, 3 major products went RTM all at the same time, there will be lots of news and more authorative blogs out there, so I'll just throw in a few pointers.
Windows 7, the next desktop OS (replaces Vista), if you haven't downloaded the beta or RC yet, I strongly encourage you to pull down one of the trial copies when they become available. Very fast, very responsive, actually runs on less resources/hardware than it's predecessor. I'm presently running my Samsung S10 Netbook (1.6 Atom Proc, comes with 1Gb Ram) with Windows 7 and Office 2010 Technical Preview (another blog post) and it's great.
Windows Server 2008 R2 (note this is what people thought would be called Win 7 Server, it's not, it's called R2). Big change here is the great new functionality in Hyper-V (Live Migration, Clustered Shared Volumes) as well as support for some great better together functionality with Win 7 (Direct Access, Branch Cache, Bitlocker to go)
Hyper-V R2, Free Hypervisor with High Availability and Live Migration capability!!!
Really good blog from the Virtualization Team detailing all the Hypervisor improvements as well as regularly updating and answering questions/statements about Microsoft Virtualization, good read here
Windows Server Team announcement here
Windows 7 Team announcement here