Holy crossed-platforms, Batman! How did I not know about Mono, the free, open-source framework that will run .NET applications on Linux and Mac OS X?
Not to get too personal, but I'm part of a mixed marriage: I run Windows and develop primarily in C#; my husband runs OS X and is not (actively) a programmer. Through love and mutual respect, we make it work. But what we have so far not been able to make work is my writing handy utilities and toys that he can use on his laptop.
I learned about Mono in Rod Paddock's intro to the May/June issue of CoDe Magazine. Then I came home, had Jon install Mono on his Mac, and gave him a quick little console app I'd written in C#. It ran like a charm. A WinForms app with one button and a popup message also ran, looking distinctly X11-y.
This is super exciting for us. We've been talking about a card-game playtest simulator, to help with his creation of card games. (Jon posts one free board game a month and has a few upcoming commercial releases.) That process usually involves a significant investment in card stock and time with the paper cutter, just to see how hands of cards come together and move through the game. A simulator would help him to vet the first and maybe second drafts of the cards without printing them out. Now that I know I can build something he'll be easily able to run, it's time to start designing!