Flow is a static type checker, designed to find type errors in JavaScript programs:

HTML_CodeSniffer is a client-side script that checks HTML source code and detects violations of a defined coding standard. HTML_CodeSniffer is written entirely in JavaScript, does not require any server-side processing and can be extended by developers to enforce custom coding standards by creating your own "sniffs".

PMD scans Java source code and looks for potential problems like:

Possible bugs - empty try/catch/finally/switch statements
Dead code - unused local variables, parameters and private methods
Suboptimal code - wasteful String/StringBuffer usage
Overcomplicated expressions - unnecessary if statements, for loops that could be while loops
Duplicate code - copied/pasted code means copied/pasted bugs