Cross-platform Custom Select Boxes with Pure CSS – Select.css
select.css is a lightweight CSS library used to consistently style the native select element consistently on most popular devices
JavaScript & CSS select
Filterable Select Box In Pure JavaScript – Asterism Custom Select
The Asterism Custom Select JavaScript library allows to create custom select boxes with item filtering support.
CSS Library For Responsive Select Element Styling
A small CSS library used to beautify the default select element that uses SVGs for the arrow symbol.
Themeable Html Select Box Replacement – select.js
select.js is a javascript library for styling the standard html select elements with CSS and javascript.