I have a list, since it's asked so many times.
They are all freebies; because i would recommend you give it try before spending money on it. Might not be your cup of tea.
http://www.blender.org/ - A fully featured model/render/postwork application with many features and a large number of available plug-ins and script add-ons. Can do everything, just about, and has some really solid free advanced renderers too.
http://www.wings3d.com - Wings3D, A polygon subdivision modeller.
http://www.anim8or.com/ - Anim8or, a modelling and animation tool.
http://www.metaseq.net/english/index.html - Metasequioa LE, Poly sub-d modeller.
http://homepage3.nifty.com/escargot/ - MarbleClay, A hierarchical poly sub-d modeller
http://www.caligari.com/Products/trueSpace/tS3/resources/default.html - TrueSpace 3.2, An older (but still solid) free version of the popular 3d modelling tool.
http://www.cbmodelpro.com - CBModel Pro, A non-linear mesh sculpting tool.

Friends don't let friends use booleans.