Things for iPhone is a well-designed, easy-to-use, highly scalable task-management app for the iPhone and iPod Touch, and it's even better when paired with the Mac desktop app of the same name. Like its desktop counterpart (with which it can sync over Wi-Fi), Things for iPhone offers a clean, intuitive interface based around the popular "Getting Things Done" productivity method. Tasks are collected under Projects (sets of smaller subtasks), Areas (areas of responsibility, such as work or family), and an In-box "waiting area" for unsorted tasks.
Things has a handy Today list (which displays the number of to-dos in the app's home page badge), a Logbook that tracks completed tasks, and categories for Next, Scheduled, and Someday tasks and projects. Every task supports added notes, URLs, alarms, and a very flexible system of tags (so flexible, in fact, that you don't even have to use them at all), and it's easy to move, edit, and e-mail tasks. By itself, Things provides an easy, elegant system for managing tasks and staying productive on the iPhone--but if you also use the desktop version of Things, the capability to quickly sync the two over Wi-Fi makes Things for iPhone an absolute must.