Compare File Extension for VS 2010/12/13/14/15
Updated 02/24/2015: Version 0.3 (Release Notes)
Control click any two files in Solution explorer and then righ click and select "Compare Files...". The configured third-party comparison tool will come up show a diff between the two files. Will also spawn a compare for subtype files if both files have subtype files, for example .designer.cs files.
Shift or Ctrl Click on Two Files, Right Click, and Compare Files
Configure Compare Files on Tools Menu
Configurable by a "Configure Compare Files..." menu item under the Visual Studio Tools menu. Configured to use Beyond Compare 4 by default at %PROGRAMFILES%\Beyond Compare 4\BCompare.exe. Tested with SourceGear's DiffMerge as well, and should work with most file comparison utilities.