Quantum GIS (QGIS) is a user friendly Open Source Geographic Information System. It has all of the common GIS functions and features. Its plugins and core features allow you to visualize maps then edit and complete printable maps. You can assemble your data and analyze, edit and manage it till you have exactly what you want. QGIS also supports many types of formats including most of those supported by OGR library, digital elevation models, landsat imagery and aerial photography. It also has a small file size compared to commercial GIS's and requires less RAM and processing power; hence it can be used on older hardware or running simultaneously with other applications where CPU power may be limited.
Quantum GIS is continually maintained by an active group of volunteer developers who regularly release updates and bug fixes. Currently, developers have converted Quantum GIS into 31 languages and the application is used internationally in academic and professional environments.
I am using QGIS as part of my Information Systems Analysis and Design module at university. QGIS was added to the module this year and it is fantastic. It turns the dry and sometimes tedious subject of database design and SQL into an emersive, challenging and enjoyable part of computer science. FANTASTIC.