Omnipay is a payment processing library for PHP. It has a clear and consistent API, is fully unit tested, and even comes with an example application to get you started.

A refreshing JavaScript Datepicker. Lightweight (less than 5kb minified and gzipped), no dependencies (but plays well with Moment.js), modular CSS classes for easy styling.

Choices.js is a lightweight, configurable select box/text input plugin. Similar to Select2 and Selectize but without the jQuery dependency.