There is all kinds of FOSS software out there. From operating systems all the way up to web server, mail server, instant message servers, cloud systems, social networking systems, blogging systems, database systems, graphical editing software, office suites, … You name it, and it probably exists as FOSS.
Not only does it exist as FOSS, but it may very well be freely available for different operating systems. This allows you to limit yourself to using free software on non-free operating systems.
In most cases this software will function identically in different operating systems, and certainly will be able to exchange its data files across different operating systems. There are two big advantages to this.
First, it allows people on different operating systems to use the same software and exchange their data amongst each other.
Secondly, it allows for people to gradually acquaint themselves with software before they switch to a free operating system, and when they do to immediately be productive and feel at ease. Typical examples for general use are OpenOffice and LibreOffice, and Eclipse and NetBeans for full blown integrated development environments.
This post is just a general introduction to what is out there as FOSS. Separate post will give more details about some of the software we use or at least know of.