JSLint Multi will use Douglas Crockford's excellent JSLint tool to check your JavaScript files for potential problems.

Drag a folder unto JSLint Multi, and it will give you an overview of the JSLint status of every JavaScript file in that folder and folders below it. Click a file in the list, or drag a file, URI or some JavaScript code unto the Widget, and it will displays details of the first problem found. Edit the file, and JSLint Multi will instantly recheck it! It even picks up new files added to the monitored folder!