Maps & GIS (File Formats): open-source projects that exist because someone reverse engineered a closed product, sorted by GitHub stars.
Geospatial format library including the reverse-engineered OpenFileGDB and MapInfo drivers
GPS log and map viewer that reads Garmin IMG, OziExplorer, TrekBuddy and many other formats
Converts between many proprietary GPS formats (Garmin, Magellan, OziExplorer, TomTom)
GIS and trip-planning app that renders Garmin IMG vector maps and reads many GPS formats
Script and reverse-engineered spec for Esri File Geodatabase .gdbtable files
Tools to inspect, split, merge and unlock-free extract Garmin IMG map files
Android navigation app that reads OziExplorer ozf2/ozfx3 raster maps
Compiler that writes Garmin .img maps from OpenStreetMap data (reverse-engineered format)
Converter for Garmin QuickDraw Contours (QDC) files to CSV tables and GRD rasters
Python library for reading Lowrance SL2, SL3 and SLG sonar logs
Decodes Lowrance SL2 sonar log files with Ruby
Decoder for Garmin IMG map files, derived from imgdecode
Linux utility for manipulating Garmin JNX raster map files
Standalone converter from Lowrance SL2 sonar logs to ndjson
JavaScript parser and renderer for Garmin IMG maps
Java ImageIO plugin for OziExplorer ozf2/ozf3 raster map files
Java viewer for Humminbird side-scan sonar recordings
Dissector for Humminbird sonar .SON recordings
Reverse engineering of the POI database format used by Locus Map LoMaps