Don't have the program for Mac OS.
There are a slew of countdown/timer widgets available for the Mac's Dashboard. You could just re-use the scap of Javascript/HTML from one of those.
...
Doh! I googled and found there was a widget for Tiger 10.4 already. It only shows how many days since you've quit, no hours or minutes. But that could easily be modified.
No Smoking v1.0 (download directly from Apple)