Originally developed by the dutch company "Not a Number" (NaN), Blender (Blender.org) is now opensource and hence continued as "free software" by the Blender Foundation, with the source code freely available under the GNU GPL license (if you want to compile Blender by yourself, here is the source).
  In short, from a user point of view, the first benefit to be aware of is that Blender is totally free for personal and commercial use. The second benefit is that Blender is continuosly developed and supported by a huge worldwide community; if you find a bug or need a new feature, you can point this out in the proper forum and, very often and in quite short times, the bug will be fixed and/or the new feature implemented.
The third benefit: due to the coding genius of Ton Roosendaal, Blender is fast, incredibly fast, both in the UI as in the scanline rendering engine. More, it has some unique feature technology that others 3D commercial packages lack: for example, its unwrapping feature has been "borrowed" and developed as external plugin for Maya (this is the point with opensource: anyone can look at the code and reuse it to make different things).
  Aimed at media professionals and artists, Blender can be used to create 3D visualizations, stills as well as broadcast and cinema quality video, while the incorporation of a real-time 3D engine allows for the creation of 3D interactive content for stand-alone playback. Blender has been used for the previz of the battle between Spidey and Doc Oc in "Spiderman 2".
In fact, Blender is a fully integrated 3D creation suite, offering a broad range of essential tools for the creation of 3D content including modelling, uv-mapping, texturing, rigging, weighting, animation, particle and other simulation, scripting, rendering, compositing, post-production, and game creation. All this with the singular benefits of a download file size of about 5 Mb and cross-platform operability with OpenGL uniform GUI on all platforms, ready to use for all versions of Windows (98, NT, 2000, XP), Linux, OS X, FreeBSD, Irix, Sun and numerous other operating systems (features and galleries here).
  You can download the latest official version of Blender here, but new builds for different operating systems are made almost every week, to test new features: you can find them on the developers forum and on GraphicAll.org.
  Documentation available on the official site here but especially on the wiki here and here for deeper explanations and lot of tutorials. Forums for questions, answers, and critique at BlenderArtists.org and news services at BlenderNation.com.

Further explanations about Blender here (english) and here (italian).