WinMerge is a nice GUI file difference engine that is freely available for Windows. This package adds support to Sublime Text 2 and 3 for allowing you to diff the last two active views, even if they are in different Sublime windows.
Install using Package Control by searching for "WinMerge".
Alternativly, bring up a command line in the Packages/ folder of your Sublime user folder, and execute the following:
git clone git://github.com/SublimeText/WinDiff.git
When you launch Sublime Text, it will pick up the contents of this package so that you can consume the goodness that it provides.
- The default key bindings file included diffs the last 2 active views upon 'Ctrl+Alt+d' (also accessible via Menu and Command Palette)