Bootstrap Zero - Open-source, free Bootstrap templates collection.
Short.io - Short.io makes short links (vanity urls) on your own domain. Free signup, analytics and API. Chrome, slack, zapier and segment integrations.
Material Design for Bootstrap 4 - Powerful and free UI KIT for the newest Bootstrap 4 and Material Design
PixelMe - URL shortener for savvy marketers
Bootstrap Magic - Create your Bootstrap 4.0 themes easily with magic
Cutt.ly - Free custom URL Shortener and branded URLs with advanced links tracking and Link Management Platform & API.