OpenLayers
Open source JavaScript library for interactive maps. MetaCarta, 2006. OSGeo project. Supports WMS, WFS, vector tiles, GeoJSON, MVT, many coordinate systems.
More flexible than Leaflet (smaller, simpler). Default choice in serious GIS development — when you need projection conversion, complex layer handling, OGC protocols. v6+ modernized to ES modules. Competitors: Leaflet (gentler learning curve), MapLibre GL (WebGL vector tiles, fork of Mapbox GL after they closed their license), Cesium (3D focus), Google Maps SDK (commercial).