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.
Upload & Convert
favicon.io is a tool to quickly create a minimal favicon for your website
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...
Bennett Feely
Bennett Feely is a web developer from Pittsburgh, currently making websites for Carnegie Mellon University.
So many good ones here too: Clippy is my personal fave