Here is a collection of utilities I've put together over the years. Largely a result of really, really not enjoying repetition and having do the same thing over and over again.
Important: All of these were designed and built to function entirely in your web browser. No information is sent to any server. You can verify this by looking in the network console of your web browser.
The closest thing to an exception is the "Connection Information" tool which does not send any information but does make an additional request to the server to retrieve the information the server already knows about you and therefore does not technically "run in the browser".