Ask HN: What libraries do you prefer for low level tasks

There are tons of interesting and different libraries out there that does tons of things each in its own way. Could you tell what libraries, plugins, gems do you use to accomplish a specific low level coding task. Provide your short reasoning for your choice whenever possible. Mine is, WYSIWYG textarea editor - CKEditor - Main for its customizablity. JS Library - Jquery - Its not lightweight, but for the overall built in features and plugings CSS Framework - Bootstrap - At first I didn't like adding non semantic classes, but the boost in productivity made me switch Authentication - Devise CMS Engine for rails - Comfortable mexican sofa - Makes no assumptions on layout Simple validation - Jquery validation plugin - Very Simple to integrate This is no way a complete list, but these are some of the library I use for Specific low level tasks. What are some of your choices.