Mad Scientist: We interrupt this celebration for a brief announcement...

Scientific Software I like

Image SXM: for analyzing SPM images (MacOS only) [freeware]
TeXShop: GUI frontend for LaTeX (MacOS only) [freeware]
IDL: Interactive Data Language... is not really a program, but a programming environment (Mac, Win, Linux) [very not free, variety of licensing options including a student version]
VESTA: Crystallographic data viewer and editor. [free for non-commercial users]
Ballview: Molecular modelling and visualization applications. [freeware, GPL]
Aquamacs: Emacs for Mac OS X, nicely done. [open source]