KMLAWSON  iFlash is now really the best out there for mac, and at a very reasonable $15 price. I used to design flashcard software and am very picky about the features since those of us who use it will use it every day.

Davey has implemented interval study - which is the key to long term study, made an application that is extremely "mac-like" and very stable (has never crashed or given me an error message to date), and offers a huge number of options but in a way that isn't disruptive or confusing - it is very intuitive to use.

The application also supports Unicode so Asian languages and other non-Roman characters are in, as well as full export and import options.

I had once planned to get back into programming to update my old Flashcard Wizard for Mac OS X, but I wouldn't want to compete with iFlash - which is really 95% of what I could wish for in a flashcard program.

For future improvements I would recommend 1) better documentation 2) smoother study review for interval study so that it cycles through cards marked wrong the first time around, without saving the results the second/third/etc. time through. While only a minor point, you have to click a button to "not save" the results on consecutive rounds and it stops removing the ones from the "pile" you get right on consecutive rounds when doing interval study - but again, minor point. This is really the best out there for Mac, and I have a feeling it will continue to get better!  
(Version 2.7.2)