I'm currently using Silo3d for making all my poser models.  For me it was a natural fit for modeling and has a good set of UV mapping tools, though I still need to use UV Mapper for some primitive mapping methods (cube, cylinder, sphere).  Silo3d is fairly inexpensive and there is a trail version that can be downloaded to allow you to try it out. http://nevercenter.com/silo/download/

For years I used Ani8or and found it pretty easy to figure out and can run off a flash drive.  http://www.anim8or.com/

I believe PhilC use trueSpace for making his Posers stuff (or at least he use to) and trueSpace can be had for free http://www.caligari.com/

I have ZBrush but never really took to it for modeling.  I do however use it for making morphs and altering models that I made with Silo.

Hexagon and Cararra are fairly popular with Poser users due to them both being owned by DAZ.  Hexagon looks fairly easy to learn but I have seen people complaining about its UVMapping abilities. 

Blender may be the most powerful free package out there But I (like many people) really find the interface hard to work with leading to a big learning curve.  http://www.blender.org/

Chris Cox