The File Comparator plugin allows you to visually compare text and binary files, showing you the differences side-by-side. It can also show detailed differences, highlighting not only lines with differences, but also the specific added, changed, and deleted characters.
From Sally 1.0.38, the toolbar icons scale with your Windows display setting, so they remain usable on high-resolution displays.
See the Menu Extension section in Using Plugins for a description of basic operations with menu extensions.