PostGIS 9.3.3 for Mac could be downloaded from the developer's website when we last checked. We cannot confirm if there is a free download of this app available. This free software for Mac OS X is a product of OSGeo project. The most popular version of the application is 1.5.
The program lies within Developer Tools, more precisely DB Tools. Postgis-1.5.3.tar.gz is the frequent file name to indicate this app's installer. Please check the Mac app with an antivirus before launch as it is downloaded from the developer's website, and we cannot ensure that it is safe.
From the developer:
PostGIS adds support for geographic objects to the PostgreSQL object-relational database. In effect, PostGIS "spatially enables" the PostgreSQL server, allowing it to be used as a backend spatial database for geographic information systems (GIS), much like ESRI's SDE or Oracle's Spatial extension. PostGIS follows the OpenGIS "Simple Features Specification for SQL" and has been certified as compliant with the "Types and Functions" profile.
You may want to check out more Mac applications, such as uDig or Thuban, which might be similar to PostGIS.
Database Tools