Developer tools are small utilities for the formatting, checking and decoding tasks that come up while building software. Formatters make messy data readable: JSON Formatter, XML Formatter, HTML Beautifier and SQL Formatter. Minifiers such as CSS Minifier, JS Minifier and JSON Minifier do the opposite and strip whitespace and comments to reduce size.
Use a formatter when you need to read or debug something, and a minifier when you are preparing it to ship. The JSON Validator tells you whether JSON is syntactically valid, which is the quick check when an API call fails. Other tools help with specific problems: the Regex Tester highlights matches as you type, the JWT Decoder shows a token's header and payload, and the Timestamp Converter switches between Unix timestamps and readable dates.
Everything runs in your browser, so pasted code and data are not sent to a server. Keep in mind that the JWT Decoder does not verify signatures, so decoding a token is not proof that it is trustworthy. Minified output should still be tested before you deploy it. If a formatter reports an error, the input is usually missing a bracket, quote or closing tag, so fix that first.