Queue of items to look into

PostGIS
Mapnik
Mapbox
OpenStreetMap
MapServer
GeoServer - Java based web server for sharing geospatial data. Actively worked on, looks like it has a company behind it.
GeoNetwork - Catalog application for managing spatially referenced resources.
MapBender - CMS for geospatial data services and map applications
MongoDB for geospatial
Google BigQuery for geospatial
Neo4j for pathfinding and graph search
Tableau

Things NOT worth looking at, that may come up from time to time