Here are some open-source GIS (Geographic Information System) tools: 1. [[QGIS]]: A widely used and powerful desktop GIS tool that offers a variety of functionality for data visualization, analysis, and editing. 2. GRASS GIS: An extensive GIS software suite that provides numerous tools for spatial data management, analysis, modeling, and visualization. 3. GeoServer: A Java-based server application that allows you to publish and share geospatial data using open standards like OGC (Open Geospatial Consortium). 4. PostGIS: A spatial database extender for the PostgreSQL database management system. It adds support for geographic objects, allowing storage and query of geospatial data. 5. OpenLayers: A JavaScript library that enables the display of dynamic maps on web pages. It provides a versatile framework for interacting with various mapping services and displaying geospatial data. 6. MapServer: An open-source development environment for building web-based mapping applications. It supports a wide range of spatial data formats and can generate dynamic maps in various output formats. 7. GDAL (Geospatial Data Abstraction Library): A translator library for raster and vector geospatial data formats. GDAL enables reading, writing, and manipulating geospatial data across different platforms. 8. Leaflet: A lightweight JavaScript library for interactive web mapping applications. It provides an easy-to-use API to display maps with various overlays and interactivity options. 9. SAGA GIS: A comprehensive geographic information system with a wide range of tools for processing raster, vector, and tabular data, as well as terrain analysis. 10. Whitebox GAT (Geospatial Analysis Toolbox): A comprehensive open-source desktop GIS tool specifically designed for advanced geospatial analysis tasks such as terrain analysis, hydrological modeling, image processing, etc. These tools offer a range of capabilities in terms of spatial data management, analysis, visualization, and application development.