Leaflet is an open source JavaScript library for mobile-friendly interactive maps. It is small in size (only 38KB of JS) and has all the features that most developers ever need for online maps. Moreover, it supports both mobile and desktop platforms. Leaflet is very good for building web map applications.
Mapbox is provider of custom online maps for websites. Mapbox GL JS (standing for Graphics Library) is a JavaScript library that allows you to create maps that can include interactive data.
OpenLayers is an open source JavaScript library used for displaying map data in web browsers. It allows putting dynamic maps in web pages. OpenLayers can display map tiles, vector data and markers loaded from any source. Additionally, it provides API for building rich web-based geographic applications.