Microsoft .NET Framework 3.5
Bootstrap version (downloads only what is missing)*: http://www.microsoft.com/downloads/details.aspx?FamilyId=333325FD-AE52-4E35-B531-508D977D32A6&displaylang=en
Full redistributable: http://download.microsoft.com/download/6/0/f/60fc5854-3cb8-4892-b6db-bd4f42510f28/dotnetfx35.exe
Excerpt from the MSDN download pages:
".NET Framework 3.5 builds incrementally on the new features added in .NET Framework 3.0. For example, feature sets in Windows Workflow Foundation (WF), Windows Communication Foundation (WCF), Windows Presentation Foundation (WPF) and Windows CardSpace. In addition, .NET Framework 3.5 contains a number of new features in several technology areas which have been added as new assemblies to avoid breaking changes."
Why use Bootstrap version?
.NET Framework 3.5 (as well as 3.0) is not a full replacement of previous version, it is upgrade that goes like this:
.NET Fx 2.0 is upgraded by .NET Fx 3.0 that is upgraded by .NET Fx 3.5, so if you have .NET 3.0 already installed you will get just few MB of assemblies!