JSONmate is a web app that can beautify (and uglify) JSON strings where these strings can be directly pasted or loaded from a remote URL.
Medoo is a lightweight (~8kb) PHP framework for working with SQL databases. The framework comes as a single file and works withВ MySQL, MSSQL, PostgreSQL,В SQLite (and more).
In order to manage MongoDB databases easily, Genghis, a single-file, self-hosted and web-based solution is pretty handy. It can be installed either as a Ruby gem or as a standalone PHP script.