SAGA -- System for an Automated Geo-scientific Analysis -- is a free Geographical Information System ( GIS ).
You can understand SAGA as a geographic information system, with a special 'Application Programming Interface' (API) for geographic data processing. This API makes it easy to implement new algorithms and exempts the developer from hassle programming overhead like user-interface design or file-io.
The SAGA API supports grid data like digital terrain models and satellite images, vector data, and tables. Since the release of SAGA there is a growing collection of SAGA-Modules, in areas of terrain-analysis, geo-statistics, image processing and process simulation.
SAGA is released under GNU Public License (GPL) Check out this new project at Sourceforge and SAGA's web site.
A GIS isn't a typical Digital Library, it is a database with spatial information included. The following excellent explanation is from a letter by Adena Schutzberg (GIS Monitor):
What makes a GIS different than a paper map is really the 'database' behind it. That 'database' includes the coordinates of roads, rivers, properties, but they may not be so accurate. Some have notoriously poor locational information actually. The database also has attribute or description information about the features - the name of the road, the depth of the river at different points along it, the owner of the parcel and how much money was paid in taxes. The accuracy of locational information and the descriptive information included are all determined by the goal of the system. This data does allow new applications because essentially you can combine the layers of data and query them in many, many different ways using spatial crieteria: "What's near nuclear power plant?" "Where is there an empty lot of a certain size that I might use to build my new warehouse?" "What's the shortest route from here to there."
A GIS contains more than just map information. It is a Location Based System (LBS), something that gets more use every year. The big jump in LBS use came when the government turned off the accuracy degrading signal in the GPS satellites. Since then, GPS chips have proliferated in phones, locators, and instruments.
A paper map can show you routes, elevations, towns and rivers. What GIS adds to this are the precise coordinates of all items of interest. This additional data, because of its accuracy, enables new applications. These range from optimal school bus routing to detailed town planning for infrastructure at the centimeter accuracy level. No more lost pipes or cables or digging to find where they are located.
While all of this is mostly good, there is a privacy implication. Many phones and a few autos have built in GPS locators. This capability has intrusive uses as well as beneficial ones, so we cannot ignore how this capability is used.
Here are a few starting points for exploring the world of GIS systems: