Visual Studio 2008 and .NET Framework 3.5 RTM
Get your new RTM bits now. I'm running Visual Studio 2008 on my box
You can also get just the .NET Framework 3.5 runtime here available as either the web setup or the full redistributable package.
This is the first time the full redistributable package has been available. It contains everything to install on any of the supported operating systems so it's large. Check it out by downloading the full redistributable and running it to unpack the individual packages. You can see the individual installers for.NET2.0, .NET3.0 and .NET 3.5 as well as for .NET 2.0 SP1 and .NET 3.0 SP1. This shows the additive nature of this release of the framework. The web setup is much more efficient for a single machine install since it doesn't even have to download parts that are already installed.
Read more about it on the .NET Framework MSDN Developer Center here.