Here's a few I find myself using quite often:

tldr | simplified, community driven man pages
Simplified, community-driven man pages!
Format JSON
JSON :: Format JSON helps to format and validate json data. JSON formatter also provides tree view which helps to navigate through your JSON data.
Analyse your HTTP response headers
Quickly and easily assess the security of your HTTP response headers

and here's where you can get a bunch more

All – Tiny Helpers
A collection of free single-purpose online tools for web developers...