First off is Make Human. They just posted an update about clothes so they are not naked any more!
This is a free alternative to Poser. It will export directly into Blender with the proper bone weights and armature.
It also exports to Collada, so you are able to use it with other applications like Maya,Poser, 3DS, Lightwave and XSI.
If you do not want to learn Blender then you can install a render engine like Pixar's Renderman, or the free Aqusis rendering engine.
Very useful for still images.
If you are not an animator do not worry.... Here is a free library of 2500 BVH motion capture animations you can use with Blender or your favorite animation package.
Carnegie-Mellon Graphics Lab Motion Capture Database
Here is a nice little viewer so you can see what the anumations look like:
They have everything from people riding horses to people swinging golf clubs.
Be sure to check out all of the shape targets in Make human, they have some for aliens and a few other monster, and animal features.
The new clothes update is in the SVN version you can read about here:
If I get a 64 bit version compiled tonight, I will post a link for windows 7 and maybe a .DEB package.