Blender can be downloaded free of charge at the Download section of this website.
Currently, there are versions of Blender available for Windows, Mac OS X and Linux. You will also need an OpenGL compatible graphics card, preferably from a quality vendor such as NVidia or AMD. As for all 3D programs, a workstation certified for 3D graphics is always best.
Blender was originally developed as in-house 3D software by the Dutch animation studio NeoGeo. As a spin-off of NeoGeo, co-founder Ton Roosendaal founded a new company called Not a Number to market and develop Blender, while making Blender available to anyone via the internet. Sadly, NaN's ambitions and opportunities didn't match the company's capabilities and the market realities of the time, and after a turbulent corporate history, Not a Number was shut down in in early 2002. (Read more about Blender's history here)
In May 2002, Ton Roosendaal started the non-profit Blender Foundation with the goal of resurrecting Blender as an open source software project. A deal was reached with the company's investors to initiate a fund-raising campaign to buy back the rights to Blender, at a cost of €100,000. Thanks to an enthusiastic group of volunteers including several ex-NaN employees, along with donations from thousands of loyal Blender supporters, the €100,000 target was reached in seven short weeks. Blender was then freely released to the world under the terms of the GNU General Public License.
Yes you can. Any creation you make as an artist with Blender is your sole property, and can be applied for any purpose you choose to. This also applies the Python scripts you write with Blender.
If you like to distribute Blender itself, you have to be aware of the GPL rules, which basically means you have to make sources for Blender available as well. However, in general it's sufficient to provide information that forwards to blender.org.
More information about the way how the license works can be found here, also with information about game files, game players and Python scripts.