FoxtrotGPS is an easy to use, free & open-source GPS/GIS application that works well on small screens, and is especially suited to touch input. It spun off of tangoGPS in 2010 with a focus on cooperation and fostering community innovation. FoxtrotGPS is freely available to the public for use, redistribution, and modification under the terms of the GPLv2. Requires gpsd.

A Python module which makes it easy to carry out GIS related operations. Look up coordinates of addresses, locations, and landmarks. Has a geocoding function. Can calculate distance between two points on the globe. Pretty straightforward to use.

A free, publically referrable, publically editable street map of the world, generated out of data contributed by the people who live there.

A map dataset in the public domain. Multiple scales of resolution available (1:10m, 1:50m, 1:110 million). Vector and raster images. Incorporates GIS attributes for mapmaking. Designed to make it easy to develop maps on your own. Comes as ArcMap MXD and QGIS documents. You can also download entire databases (even SQLite) all in one go.

A FOSS GIS software package. Create and edit geospatial information on just about every platform. Maybe useful?

Nominatim is the OpenStreetmap server for searching the database for an address and figuring out where in all of the map datapoints that address is. When you figure out where in the database it is, you can then serve up the appropriate map tiles. Powers the search box on the website. Used to import the map data.

The public github account of the NSA.