Visual Studio Tools for Unity is a free Visual Studio add-on for working with the Unity gaming tools and platform from Visual Studio 2010, 2012 and 2013.
Formerly known as UnityVS, this add-on that provides Visual Studio's particular programming and debugging experience to game developers working with the Unity platform. It was originally developed by Jb Swain at SyntaxTree, a company recently acquired by Microsoft with Swain becoming Senior SDE Lead in the Visual Studio Platform Team.
The Unity 1.9 release is the first from Microsoft and its highlights are:
Faster debugger. Attaching and detaching the debugger as well as expanding local variables is now faster.
Faster startup. Opening VSTU projects is now faster.
Better handling of C# constructs. The local variables window is now properly populated when debugging iterators or when variables are accessed inside closures.
Start your game and your debugging session in one click. This feature had been one of the most-requested: you can now attach the debugger and start the game by simply changing the debug target. This is only available in Visual Studio 2012 and 2013.
Writing on the MSDN Blog, Dave Voyles points out the UnityVS is free for all developers using Visual Studio and invites devs who don't have Visual Studio to contact him for a free BizSpark account, which includes Visual Studio Ultimate 2013 and Azure credits. Note that you cannot use the plugin with Visual Studio Express it has to be at least VS Pro.
The idea that you could work with Unity in Visual Studio wouldn't have seemed a reasonable idea until quite recently. It is evidence of change at Microsoft and another validation for Unity as a 3D game creation tool.