What is a good GTD app with cloud sync on iPhone, iPad, and Mac? I am looking for a good GTD (todo/task/project management) app that has cloud sync. I'd like it to be on my iPhone, iPad and Mac. Web app would be nice, too.

OmniFocus is the closest thing I've found but it seems too expensive.

Things don't have clound sync... only WiFi sync is available for Things.

Do you know of any GTD app I'm looking for?
Thanks!
Check Wunderlist by 6Wunderkinder: http://www.6wunderkinder.com/wunderlist/

It's free. Iphone and ipad apps are available on itunes app store.