Is there a project management (GTD) app that integrates with iCal?

I have been searching for a GTD app that would integrate with iCal. It seems pointless to me to have two separate calendar systems if I am trying to stay on track (which is what most of the GTD apps seem to offer). iCal is too limited on it's own: can't make daily lists; can't pull up different project check lists; project time lines etc. Any suggestions?

Personally, I am not crazy about iCal, but I use iCal because it seems to integrate best with the iPhone and Mac Book. Would rather use Gcalendar, but the integration was not great.