If you have switched to the Mac because of its Unix underpinnings and elegant GUI, then you aren't alone. If you want to install open source packages on your Mac, then you have at least two good options --
MacPorts and
Fink. Installing
Fink not only gives you access to a large number of open source programs, but lets you track which programs are installed on your computer, handles dependencies, and helps you to upgrade with a minimum of fuss.