What started a short list became quite a big description, more details inside the post.
Mailcatcher - runs a super simple SMTP server which catches any email message sent to it to display in a web interface.
php-queue - unified front-end for different queuing backends. Includes a REST server, CLI interface and daemon runners.
pesticide.io help to find CSS bug by placing an colored outline on every element on a page. Also available as Chrome extension.
SuperHeroJS - great collection of articles on all aspects of JavaScript. Starting from very basics up to questions of advanced usage, code organisation, testing, tools and performance
Awesome library for dynamic CSS shadows shine.js. Dynamic light positions, customizable shadows, uses text or box shadows based on content