Daz3D is a piece of software that you see people use all the time on sites such as Deviant Art. Think of Daz as the free version of Poser. They can use the same resources and damn near look the same...It has pre-made human models that you can purchase add-ons, such as clothing, hair-styles, etc. for. It is fairly easy to use as all you need to do is manipulate a model into a pose and render. Fairly easy but it takes a bit to get a good render. A good piece of software for those just starting out and interested in making some 3D art. Some safe for work examples of what you can do with this package are in the below links.
http://browse.deviantart.com/digitalart/3d/characters/female/?order=9#/d1gvl1ahttp://browse.deviantart.com/digitalart/3d/characters/female/?order=9&offset=96#/d1yy5wthttp://browse.deviantart.com/digitalart/3d/characters/female/?qh=§ion=&q=wonder+woman#/d3ad0cs...and the absolute god of Daz/Poser is a gentleman named Fygomatic, he manipulates the default models that come with Daz and remodels the faces to look JUST LIKE famous models and actresses. His Jessica Alba is AMAZING! This link is for his Deviant Page...be warned some of his images NOT safe for work.
http://fygomatic.deviantart.com/Sculptris is another freebie, that comes from the maker of ZBrush, which is a 3D modeling software based upon the idea of sculpting a ball of clay. ZBrush is currently used by many game studios for their character modeling and Sculptris is their free version (with a different interface) of this program.
Blender is another freebie that is a full on 3D app along the lines of 3D Studio Max, Maya, Softimage, Lightwave, etc..., but it is free.
Speaking of 3D Studio Max, Maya, and Softimage. You can download 30 day trials for both Max and Maya (Lightwave also has a trial, but it may only be for 14 days, not sure...) and as for Softimage, you can get the
Softimage MOD Tool which as discussed on the
Valve Developer Community Website you can download and use to make modifications to Source Engine games without restrictions. There are some limitations to what the MOD Tool can do, but it is a free tool to learn how to mod games.