Extensions or Add Ins save application development time and make smart code. Visual Studio has vast list of these extension available. I am giving brief about some very useful and powerful extension here, but before this you need to know how to install these extensions.
How to Install Extensions
- Open Visual Studio.
- Go to Tools->Extension Manager.
- You will find Installed Extensions and Online Gallery of extensions.
- Download extension.
Indent Guides
This extension adds vertical lines at each indent of code. It is very useful to maintain indent in code.

You can read more about this extensions at http://indentguide.codeplex.com/.
VS10x Code Map v2
This is code visualizer extension that displays a graphical nested representation of the current code editor window (C# and VB.NET). It helps the developer visualize type nesting, implemented interfaces, regions, member type and scope, as well as quickly navigate to their respective positions in code
There are some good features of this extension:

You can read more about this extension at http://visualstudiogallery.msdn.microsoft.com/1c54d1bd-d898-4705-903f-fa4a319b50f2?SRC=VSIDE
Visual Assist X
This is one of great extension which reduces development time with key features like refactoring code, fast search, instant navigation etc.

You can read more about Visual Assist X at http://visualstudiogallery.msdn.microsoft.com/D6678A1F-C700-4532-8C5E-A799D875063D?SRC=VSIDE
AutoCode 2010
This is my favorite extension which really reduce my development effort. It can generates more code by less typing. There is enough documentation available to use this.
Features
- More than 100 useful commands.
- Commands invoke on ctrl+Enter.
- Construct class, common methods with single line of code.
- Create regions of code blocks
- Create summary block.
- Support custom commands.

You can read more about this at http://www.devprojects.net/
Hopefully this information will help you in your development.