Turbosquid is likely the most popular 3d marketplace online and although a large amount of the models costs money, there are a ton of great free assets. All you need to do is create a free account and start downloading. There is no real theme in terms of the kind of models are offered because it is an open marketplace. All the models are third party creations that you can download. Conversely, if you are a 3d modeler, it is a great place to sell your models.

Blender is a free alternative to commercial 3d packages. It is capable of many of the same things a commercial package would give you, including modeling, texturing, particle effects animating, video editing and rendering. Although it is a free piece of software, it is very powerful and continues to improve its interface and documentation. An interesting feature is that it also has a game engine component and features Python scripting. Blender is embraced by a vibrant user community and has many great free resources available to help you learn.

Sculptris is currently a free mesh sculpting program which has recently been acquired by Pixologic, the makers of zBrush. Sculptrus is in some ways similar to zBrush and supports a workflow to continue developing in zBrush if you choose to do so. One nice feature of the program is that it attempts to remove complexity and some of the technical concerns that can inhibit an artist. Instead it provides you with an intuitive ability to sculpt digital clay.

There are a number of freely available game development environments floating around but Unity stands out for a couple of reasons. First, there is a free version. This means anything you create with it is yours to sell without having to fork over any share of the revenue back to Unity. It is also cross platform so you can develop on Mac OS or Windows as well as deploy to those platforms. It is easy to use and you can get started in it quickly. Things you build in Unity can also deploy to a web browser so it is ideal for creating 3d web content.

Unity also hosts an Asset Store where users can download game assets, many of which are free. The Asset Store is also a good place to give away or sell assets that you have developed. Anything from scripts to 3d models can be found on this online marketplace and can easily be imported into your game project.