Life Balance An elegant to-do list manager that adapts really well to the GTD approach while using clever algorithms to help balance your life's priorities. Not cheap at $19.99 (plus more if you want the desktop version), but worth it.
Pocket Informant LITE (Calendar, Todos) I like this calendar MUCH better than Apple's default. It syncs with Google calendar and includes a To Do list function that syncs with a cloud-based system (Toodledo). Paid version looks even better.
Jott Leave yourself voice messages which are transcribed to text. I really liked it when it was a free service, and for those willing to pay, I assume it's even better.
Remember The Milk I prefer the overall approach of Life Balance, but to-do list manager RTM wins points for its web-based service.