If you'll be doing development on it, grab XCode, for sure. You may also want X11. Apple provides both as a download.
Lots of people really like TextMate as an editor.
For standard unix apps and free-as-in-freedom software from the Linux/BSD world, you have two options: Fink (http://www.finkproject.org/) and Mac Ports (http://www.macports.org/). Which one you go with is largely a question of whether you prefer apt-get or a BSD-style ports tree. I have heard that Mac Ports has some nonsensical dependencies (requiring Gtk for a KDE install, for instance) but this is third-hand.
no subject
Lots of people really like TextMate as an editor.
For standard unix apps and free-as-in-freedom software from the Linux/BSD world, you have two options: Fink (http://www.finkproject.org/) and Mac Ports (http://www.macports.org/). Which one you go with is largely a question of whether you prefer apt-get or a BSD-style ports tree. I have heard that Mac Ports has some nonsensical dependencies (requiring Gtk for a KDE install, for instance) but this is third-hand.
Adium for IM.