Gimp, OpenOffice, xfractint, mrxvt, fontforge, more
I like Gimp a lot and it is lot more stable on OS X now than when I initially used it. It keeps getting better. I also got a very good response when I couldn't get an update to compile - good enough that I could compile, though an update including the fix soon followed. (You don't need to compile it yourself - certainly don't try unless you have experience compiling software and want to for some reason.)
OpenOffice has improved by leaps and bounds on OS X. I have had much better luck with the X11 version than with NeoOfficeJ and like it a lot better. I don't prepare my own documents in it though - I only use it for dealing with documents from others. Actually, I do produce spreadsheets with it and have been very impressed by its compatibility with Excel. I have quite complex spreadsheets when I have teaching assistants - a master spreadsheet picks up data from per-ta spreadsheets etc. I also automate a lot of the calculations, use charts etc. I've had very few problems sending the tas spreadsheets back and forth whether the tas are using Excel on Mac or PC. I did have one issue, but this was resolved once we figured out exactly which format I needed to export in.
xfractint is a port of the DOS programme fractint. It isn't as stable as the DOS version, but it does have a graphical interface. It is one of the oldest fractal programmes. Currently, it compares very favourably with some of the other programmes available in terms of power and flexibility. It is not, however, a good application to begin exploring either fractals or X11 with. I'm still trying to figure it out - it isn't intuitive at all.
mrxvt is installed on my Mac (so it is available for Mac now), but I still like Terminal better for most things and I prefer it to iTerm. I run screen instead of using a terminal programme which includes tabs itself.
fontforge is a font editor, converter, viewer, creater etc. Very useful if you ever need to convert fonts from one format to another (e.g. to use a font with TeX).
I'm not sure by what criteria Apple's X11 is supposed to be better than the alternatives. I would say it depends on what you need to do and what version of OS X you are using. Tiger uses a different version than the previous OSs. Apple's X11 is odd in various ways and still not as stable as I would like. It is also not very up to date. I still use it, but if I needed an application which relied on menus etc. being available in the root window, I might not. I've not had good luck using Apple's X11 in anything but rootless mode. The window manager integrates quite nicely into Mac OS X, but copy/paste isn't as well implemented as it could be (and has been in other implentations).
I'm not personally a fan of fink but, on the other hand, it is a long time since I tried it so I expect it has improved drastically in the meantime. Ditto for macports (was darwinports).
- cfr