PostGIS

PostGIS is a spatial database extender for PostgreSQL object-relational database. It adds support for geographic objects allowing location queries to be run in SQL.

In addition to basic location awareness, PostGIS offers many features rarely found in other competing spatial databases such as Oracle Locator/Spatial and SQL Server. Refer to PostGIS Feature List for more details.

For PostGIS downloads and installation instructions follow this link.

QGIS Geographic Information System

QGIS is the best and most powerful GIS tool in the free and open-source software (FOSS) community. Like any of its commercial competitors QGIS can create, edit, visualise, analyse and publish geospatial information. It can open numerous popular file formats, including shapefiles, imagery, digital elevation models and spatial layers on MySQL, PostgreSQL, SQLite and many other databases.

There are a huge set of plugins for data querying, manipulation and production of new layers available for QGIS which are easily installable from the Plugins menu. These, together with QGIS, make expensive commercial GIS systems with similar extensions inaffordable.

QGIS is available for Windows, Mac, Linux, BSD and Android and can be downloaded from here.

ArcGIS - ArcMap

ArcGIS is the most widely used desktop GIS system to compile and manage data, work with advanced maps, perform spatial analysis, and conduct GIS projects.

It has many extensions, most of which are commercially available, to produce high-quality maps for publishing on paper or the Internet.

You can download a 60-day trial version from ESRI's website.