I started to use Blender in 2013 and I was amazed by its tool set and the development. It is not only cool for a free piece of software, it is cool, period! The more I learned, the more I realized how well it works in many areas, for example character tools, animation, modelling, sculpting, re-topology, UV unwrapping and texture painting.
Open source software has a big advantage besides being free. Typically it takes a lot of time to learn a software package well enough to use it in a professional environment. That is the big investment, much bigger than the actual money you pay for the package. So from the professionals point of view the fact that a software package is free isn’t as big a deal as it is for a student or a hobbyist. The real advantage is that there’s no PR department deciding where to go with the software on the basis of what’s selling best. There’s no company owning and maybe selling the package, leaving you stranded in an dead end with all your precious knowledge and experience. This means learning open source software is a safe investment.
Other interesting open source projects: Krita (a very cool painting program), Gimp (image editor), Inkscape (vector graphics) and Natron (compositing).