The Open Source Geospatial Foundation (OSGeo) is a non-profit non-governmental organization (NGO) whose mission is to foster the use of geospatial technology and data. OSGeo does this through providing open source packages that are easily downloadable or booted onto a computer and contain a myriad of softwares. The foundation also encourages community involvement; members can contribute their own resources for cross-project collaberation in a way that will benefit the public. This is achieved through public forums, chapters, events, GitHub, and more. The OSGeo website also contains a section of third party tutorials that cover everything from getting started with QGIS to processing specific data. Many of these tutorials are available in other languages.
There are many packages available on the OSGeo website and users can choose based a variety of factors: what softwares are contained, the operating system one is working with, how one wants to process data, and what kind of technology is relevent. OSGeo makes finding a suitable package or stand alone software easy.
General overview: https://en.wikipedia.org/wiki/Open_Source_Geospatial_Foundation\