Sketchup Free and Pro are quite good and have a very "free flowing" build style.  The Pro is $500.00 and the Free version has a proprietary  file format -( a quick google search and I found a few "obj" exporter plugins). - excellent for architecture 

Wings3D sorta looks like a toy but underneath is a very powerful modeler that promotes good 3D discipline.  There are a number of plugins available including one that allows you to load in vector graphics and covert them to 3D objects -  excellent for architecture and box-modeling

Silo 2 is my modeler of choice - when using Silo I feel very "connected" to what I'm working on.  However, development has dwindled and that can be discouraging.  Good for architecture and excellent for character creation.

Take care