You can make a program multi-platform by programing it in a multi-platform language.
I am not advocating Java, as it is the slowest, most bloated, pile of......
ok, calming down now...
There are various good, fast languages that have good and bad points.
perl, you need a perl interpreter and the appropriate libraries.
python - see perl.
java - see above.
Ajax with an active apache server controlling it - probly the best but would need a centralized server.
Pascal - I would recomend this as the best overall method. Anything you write can run on Windows (all versions) Mac, linux, bsd, gameboy, openzaurus, xbox and a few other ports.....
It is a standalone compiler that you specify the target platform and it produces a executable to run on it.
It is free, as is all the documentation.
It has a visual developement language. (drag and drop visual design)
Did I mention it is free.
www.freepascal.org
lazarus.freepascal.org
If you want some software written, I can probly knock it off quickly for you (won't be preaty, but it will be functional).
best regards
Dalton