"The GeoNames geographical database is available for download free of charge under a creative commons attribution license. It contains over eight million geographical names and consists of 6.5 million unique features whereof 2.2 million populated places and 1.8 million alternate names. All features are categorized into one out of nine feature classes and further subcategorized into one out of 645 feature codes. The data is accessible free of charge through a number of webservices and a daily database export. GeoNames is already serving up to over 11 million web service requests per day. "
Users can switch maps as desired based on personal taste and quality of maps in their local area. Various tools built on top of Mapstraction allow users to easily integrate maps into their own sites, and configure them with different controls, styles, and provider. "
Geocoder.us is a popular free geocoding service for addresses in the United States. The site uses TIGER line data along with geocoding code written in perl to provide geocoding services for thousands of websites. The site's creators also provide paid geo-related services.
MapServer is one of, if not THE granddaddy of the open source GIS tools. Developed for NASA by the University of Minnesota in the 90s, MapServer was adopted by the OSGeo and development continues today. MapServer is cross-platform and MIT-licensed, so it's used by tons of organizations and projects.
MapServer serves maps - it can generate and serve maps based on tons of input, scripted associations, custom development, base layers, output technologies, etc. It is not a tool for geospatial queries and analysis, however.