QGIS is an open source option for GIS software. There are many benefits to using QGIS including:
The latest version of QGIS is 2.18 (September 2017) and the software is updated often.
Lynda.com: Learning QGIS
A 2-hour video tutorial introducing users to QGIS (Duke users only).
User Guides (online documentation), Training Manuals (tutorials), and A Gentle Introduction to GIS (overview of GIS principles and terminology)
Open Source GIS with QGIS 2.0
Tutorial from Harvard's Center for Geographic Analysis (CGA). Pretty thorough, with lots of screenshots.
MMQGIS plugin package
Documentation on the useful plugins that come in this package. The package can be installed from the Plugins ... Manage and Install Plugins... menu in QGIS. Turning it on will add an MMQGIS menu to the interface.