Leopard only, and wow. It is lightweight in deed.
For what it’s built to do it looks like it does it perfectly. Add a to-do item in Anxiety and it puts it in your Mail and iCal to-dos as well. Perhaps not the ideal application for the heavy GTDer but probably a god-send for the average, “I need help keeping track of my things to do”er.
And it’s free. Hello.