Online:
Regex101 – useful for testing your regex patterns
jsFiddle – test your html / java and css configurations
dotnetfiddle – test your .Net code
hexed.it – online hex viewer
fishbowlie – online FPS test for your browser
https://stackedit.io/ – Really cool online markdown editor
https://www.unitjuggler.com – great tool for converting different units
Offline:
Spacesniffer : which I find the one of best tools to quickly see what is taking space on your drive
Etcher – Great multiOS tool to burn out bootable ISOs into USB. Simple and effective