http://www.asjava.com/tools/fiddler-tutorial-how-to-use-fiddler/
Illuminations and firebug: it’s aware of popular JS frameworks such as Ext.js, Angular, etc.
DBVisualiser: allow you to edit rich text and images in vast amount of databases. http://www.dbvis.com/
RegExr: to test and visualize regex: http://regexr.com/
XPath Visualiser: to test and visualize XPath for XML query: http://xpathvisualizer.codeplex.com/
WinMerge: to compare files to find differences.
Visual Studio Web Essentials plugin: add useful features for JS development such as #region