March 2005 "Avalon" and "Indigo" Community Technology Preview
If you want to play with the March 2005 "Avalon" and "Indigo" Community Technology Preview, head on right over to the MSDN Subscriber downloads website (FYI, the .iso file is 444 MB). You have a few options once it's installed - you can try coding everything command line like Sam Gentile. Or opt to use the February CTP of Visual Studio 2005.
Next, you need documentation. A really good place to get started for all things Indigo is the Longhorn Developer Centre. David Chappell has written a really nice primer to get your head wrapped around the new concepts. Then head on over to the FTPOnline website for Indigo webcasts from VSLive! (I'd highly recommend looking at the Steve Swartz/Don Box webcast with a soup to nuts overview of Indigo. Don also has a very informative weblog with great tidbits of Indigo here and there).