I am happily a full-on Mac users these days. I still keep a window box at home and at the office to do the kinds of things that remain only possible on a windows box due to specific software packages: ArcGIS, SolidWorks, ERDAS and well, that is about it. Increasingly I am finding new ways of doing even these things on the Mac. For example, QGIS is an excellent (and free) GIS front end to GRASS.
Some of the programs I am finding for the Mac are really gems that make using OS X a techie dream. For example (and this doesn't include the world of X Windows based software):
Plot: http://plot.micw.eu/Main/HomePage This is a great free program for doing XY charts with lots of controls for error bars, perl based scripting, etc.
Octave: http://www.gnu.org/software/octave/ An incredible package for solving linear and nonlinear problems numerically using a Matlab language - all for free. See also: http://sourceforge.net/project/showfiles.php?group_id=2888&package_id=237839
VVIDGET: http://www.vvidget.org/ The rather incredible VVIDGET application that allows one to drag and drop all kinds of features into 2D and 3D graphs. Something ya gotta see...