This collection of tools adds IntelliSense and debugging features to Visual Studio; if you use Visual Studio and Unity, these are a must have:

• IntelliSense: An intelligent code completion tool for Microsoft Visual Studio. This is designed to speed up many processes when writing code. The version that comes with Visual Studios tools for Unity has Unity-specific updates.
• Debugging: Before this extension existed, debugging Unity apps proved to be a little tedious. With this tool, we can now debug Unity applications inside Visual Studio, thus speeding the bug squashing process considerably.