What is in .NET 3.0?

Following things are present in .NET 3.0:

1. .NET Framework 2.0
Common Language Runtime (CLR)
Base Class Library
ADO.NET 2.0, ASP.NET 2.0, Windows Forms 2.0
VB 8.0 and C# 2.0
2. WinFX
Windows Communication Foundation (WCF)
Windows Presentation Foundation (WPF)
Windows Workflow Foundation (WF)
3. InfoCard
Renamed to Windows CardSpace (WCS)

WinFX is useful to build SOA(Sevice Oriented Architecture), n-tier, etc...
IMP: .NET 2.0 Framework and compilers are unchanged, so your existing code will continue to run as it exists today in .NET 3.0