TaskAgent

View and edit your to-do lists from anywhere. Because TaskAgent syncs your lists as plain text files through Dropbox, you’ll always have access to them. But if you just wish to keep your lists in the app, that’s okay too.

TaskAgent features:
-Interactive list interface: simple and easy to use
-Dropbox sync: your lists will always be there for you
-Dropbox editing: open the lists with any text editor and have the changes synced back to the app
-Share your lists: copy them as text, send them as an email, tweet a link, send a text message, or to other compatible apps.
-All tasks can be assigned an optional note
-Support for multiple lists
-List archiving option
-Task sorting options
-Landscape mode support for the iPad
-Discrete option for the iPad: hide list previews on the sidebar
-Supports TextExpander touch snippet expansion

Also available: TaskAgent for Mac

What people are saying about TaskAgent:

“The best solution to manage lists on iOS while retaining the capability of editing them anywhere as long as I can open a .txt file.” — Federico Viticci, MacStories

Winner of the App Showdown for Organizing iPhone Lists — Christine Chan, AppAdvice

“I use [...] TaskAgent for iOS every day.” — Dan Frakes