While not a full GIS, Google Earth is a good way to get started quickly with some impressive spatial visualizations.
The web version of Google Earth can support many basic mapping projects, and Google Earth Pro is available for free download onto Windows and Mac machines and offers more advanced import/export features as well as historical data.
QGIS is a well-established free and open-source GIS platform that works on both Windows and Mac machines.
ggmap package for R/RStudio
If you're working with R/RStudio and including geospatial data in your quantitative analysis, you can download the ggmap package which includes a collection of functions to visualize spatial data and models on top of static maps from various online sources (e.g. Google Maps). It includes tools common to those tasks, including functions for geolocation and routing.