AddIn are great to improve productivity in VS. I myself use couple of Addin.
Every AddIn would have got it's own set of functionality and it's own window's. Specifically for Javascript Parser, you can open it by navigating to View->Other Windows -> Javascript Parser in menu bar in Visual Studio. I am attaching a screenshot from my system.

Link for couple of recommened VS AddIn.
UPDATE 1:
AddIn provide extension to VS, they are not limited to one window. So a AddIn like Resharper (see in attached Image) is in Menu Bar. Web essentials comes into work when you a open a css file which has link to image etc. So every AddIn would extend VS in certain way. How is it to be used, should be documented in AddIn documentation.
Quote From VisualStudio.com
Visual Studio was built for extensibility to expand the functionality
of the IDE. Here are some examples of how you can extend the Visual
Studio IDE.
Add new UI such as menu items and tool windows to add your own tools.
Extend the editor and our compilers to analyze and understand your
code.
Extend Visual Studio Online and Team Foundation Server to
analyze information, manage the development workflow, or integrate
external development services.
Integrate a new compiler and project type into Visual Studio to bring new assets into your solution.
Distribute your tool with the Visual Studio shell even if your users
don’t have Visual Studio.
If you need more details please go through 1 & 2 & 3