I'm currently experimenting with flashcard style programs on the iPhone that used spaced repetition in order to remind you at exactly the right time. These apps can be used not only learn languages, such as Japanese, but to reinforce good habits (exercise, chores etc) that don't need a sting like Beeminder delivers (see below), nor to have loads of reminders clogging up your phone's lockscreen. Spaced repetition is great for habit reminders as Anki Notes offers you different intervals measured in days depending on how well you felt you remembered the habit.
Beeminder is a web application (and iPhone app) that fines you real money if you don't stick to your commitment. For example, you can commit to writing a certain amount of words per day, or do a certain number of steps, and if you don't meet that commitment you will be fined say $5 (you can choose an amount more than this). That will horrify many people, which is exactly why Beeminder works. The desire not to be fined makes you complete the task like no other app can.
A powerful todo list that allows todo items to be nested as deep as you like, which makes it great for brainstorming and rearranging your ideas before checking them off as you complete them. Lots of powerful filtering options available to display your list in different formats, such as What is the Next Action.