as well as versions for other OSes, including Windows, linux, Solaris, Irix, OpenBSD, and others. Yay for open source!
http://www.blender.org/download/get-blender/You'll want one of the 2.5x builds. 2.49 has a different interface and isn't as streamlined as 2.5x, and even though the 2.5x branch is considered "testing", it's what they made the "Sintel" short film with. 2.5 is just a better, more professional representation of the software, and is quickly replacing 2.49.
You may or may not also want Python installed, depending on if you'll be messing around with custom Blender scripts; the Python API is tightly integrated into Blender, and is the language used for all Blender scripts. Python is optional out of the box, though.
Blender's feature set includes a suite of tools similar to those found in other commercial packages, including rigid and soft body dynamics, a built-in fluid solver, a built-in game engine, an IPO curve editor for animation, a node-based texturing tool (kind of like what Maya uses), modifier-based modeling tools, and support for external renderers such as LuxRender. It also provides a mudbox-like sculpting tool, and although that still needs some improvement, it's definitely useful enough for beginners or those on a budget. Blender is capable of providing a solution for each point in the pipeline, from rough cut visualizations to the finished, fully-textured and animated end result, and also has tools for compositing the final product into film. Just be aware that Blender's interface takes a little getting used to and, as you may expect, hotkeys are king.
Three animated short films have been made to help show off Blender's capabilities, as well as to help the people at the Blender Foundation further develop the software. These are titled "An Elephant's Dream", "Big Buck Bunny", and "Sintel", and can all be found on YouTube.
A truly excellent resource for learning Blender are the video tutorials provided at Blender Cookie (www.blendercookie.com), in particular the tutorials done by Jonathan Williamson (sp?). He almost always says which hotkeys to use for whatever operation, and usually goes into detail regarding the "why" of what he's doing.
I've dabbled with Mudbox a bit and watched some timelapses on YouTube, and I must say I'm impressed. I'm not a character modeler (yet), so Mudbox doesn't have a lot of use for me at the moment, but it's certainly versatile enough to be on my shelf should I ever get that far.