GRASS (PC/MacOS X/Unix)
A relatively complex GIS with raster, topological, vector, image processing, and graphic production functionality. It is open source and free.
Idrisi is a raster GIS/Remote Sensing software that specializes in raster data. A suite of tools provides capabilities for image processing, restorations, enhancements, and transformations.
LandSerf (PC/MacOS X/Unix/Linux)
LandSerf is a freely available GIS for the visualisation and analysis of surfaces. Applications include visualisation of landscapes; geomorphological analysis; gaming development; GIS file conversion; map output; archaeological mapping and analysis; surface modelling and many others.
Manifold System (PC)
Manifold System is a relatively cheap GIS that support different types of data and offers a wide array of tools. It may be used to produce shaded relief and perspective views among other things.
QGIS (Linux/Unix/MacOS X/Windows/Android)
QGIS is a user friendly Open Source GIS and supports numerous vector, raster, and database formats and functionalities.
Surfer is a contouring and 3D surface mapping program. It converts your data into contours, surfaces, wireframes, vectors, images, shaded relief, and maps.