I always replace notepad with Notepad2-mod on Windows.
Heavier editing goes to Sublime Text or Visual Studio/Xcode.
TortoiseSVN on Windows, Cornerstone 2 on macOS for subversion. (Not paying for Cornerstone 3 when they can't even be bothered to update Subversion past 1.8). GitHub for git on both.
Acorn for image editing on macOS, I prefer it to Pixelmator. Paint.NET on Windows although I'd like to find something more equivalent to Acorn on Windows in anyone has any suggestions.
StartIsBack+, Display Fusion, f.lux, Audio Switcher and Fences to improve my Windows 8.1 experience.