Developers like to have extensions that make work easier and faster.Here is list of extensions I found very useful-

 
Code Digger

Based on Pex, this extension analyzes possible executions paths of your code and tries to find possible edge cases and exceptions. Download it from visual studio gallery.

Web Essentials

Visual Studio 2013 has already a lot of HTML/CSS/JS editing features, but Mads Kristensen’s Web Essentials adds even more: zen coding, JSHint, LESS, SASS, CoffeeScript, Markdown, image optimization, intellisense for CSS Media Queries, robots.txt files, HTM5 appcache, AngularJS tags and much more.

Basically an essential extension for anyone doing modern web development using .NET.

CodeMaid

CodeMaid is an extension that helps cleaning up and organizing your code. Useful even if you have already ReSharper… and vital if you don’t. Have a look at the documentation to see all that it does.

Productivity Power Tools

From Microsoft, this extension adds some pretty useful features that help you navigate through the IDE faster. Some duplicates features available in ReSharper, but others are unique. So, install it even if you have it. Check features and download it from the visual studio gallery. And there is also a video that explains the new features.

Routing Assistant

Not an extension for everyone, but very useful if you do ASP.NET MVC - it explores your solution and visualizes in a tree-like interface all the routes and acts also a routing debugger to help you see which routes are called by specific URLs.

It’s a free product from a software company that commercializes ASP.NET MVC libraries.

SideWaffle

Going along with Web Essentials, SideWaffle adds a bunch of project and item templates and snippets for modern web development (and coming also Windows Phone and Windows Store).

Nuget

Another extension that doesn’t need an introduction is Nuget. It ships with VS2013, but for older versions of Visual Studio you have to install it manually.