LINQ

I've recently been getting very excited about Microsoft's LINQ project. It brings SQL style declarative query programming into the .net framework and makes it a first class feature of C# and VB.NET. The overview by Don Box and Anders Hejlsberg is here and there's an excellent overview of the current state of the object/relational problem and how LINQ will help by Ted Neward here. There's also a great Channel 9 video of Anders Hejlsberg explaining LINQ.