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Here are the extensions I use, and strongly recommend you try. I have seen no adverse problems from running any of these extensions. Here are the extensions I use:
Productivity Power Tools (by Microsoft; free):
For VS2010: Adds a Solution Navigator, a better Solution Explorer, and cool tabs features like pinning and color coding by project.
For VS2012: I haven't explored yet, but will update
PowerCommmands Visual Studio (by Microsoft, free):
For VS2010 Only: Several random features, the ones I really enjoy are Open Commmand Prompt here and Open Containing Folder.
NuGet Package Manager (by Micosoft, free):
A package manager for .net. Say you want to use an open source library in your VS project, you open package manager and say Install-Package . With that the project you requested along with it's dependancies are downloaded and liked to your project. Must have when working with popular .net projects.
Resharper (by Jetbrains, 199$) This is "the" extension for visual studio. If you haven't used it, get the trial, I suspect you'll conclude it's worth paying for. Resharper has several strong refactorings, plugins to fix stylecop errors, and first class support for running NUnit.
VSVim (by Jared Parsons, free)
If you don't love VI you skip this. If you love VI you get a fifth must use extension. VsVim gives VI keybindings to visual studio. It's being actively developed and gets better every release.
Again, I strongly recommend if you use Visual Studio that you try out these extensions right now, trust me, it’s worth the effort.