I use almost all free software. I do all of my work under the MacOS unix environment with vim, gcc, python, etc etc. You can use the MacPorts ( http://www.macports.org ) package manager to install and manage your usual unix utilities. (MacPorts is analogous to portage/emerge on Gentoo, apt-get on Debian, yum on RedHat/Fedora, and FreeBSD's ports). I also use Firefox for web and Adium X for IM.

The only thing I can recall paying for is Parallels, a virtualizer similar to VMWare, to run MS Windows under MacOS. I hardly ever use it.