Clear is a simple to-do app for the iPhone and iPod touch ($0.99 in the App Store) that announced it had sold 350,000 copies in its first nine days on the App Store.
A great looking app that at the price is a no brainer if you're in the market for a simple to do app1. Much better for most tasks than Apple's own reminders app, that's for sure. And with an iPad and Mac version on the way, you'll be able to keep things in sync if you need that kind of functionality.
Demo Video of Clear permalink
Direct link to video