Non intrusive lightweight plugin that adds an option on right click -- only with selected text (Selecting the whole number is necessary) to convert that number from a unit to another unit, or currency to currency or anything else right on your web page just changing the text in front of you. The only requirement for the page is that you are selecting a number (Trailing characters on either side are usually okay though).
No information is tracked, the plugin is customizable (add your own conversions, add your own currency conversions, remove currency conversions)
Feel like you have something to add? This extension is open-source and on github - https://github.com/ryanr1230/KingConverter. And if you have a useful addition that you want to make publicly available, I will probably accept merge requests. Even if not the code is open-source, and you can see for yourself that none of this is storing any information you don't want it to.