I'll add my vote for Things! I was designing my own app to do this then Things came out. If you've ever been inspired by GTD - Getting Things Done, by David All

I'll add my vote for Things! I was designing my own app to do this then Things came out. If you've ever been inspired by GTD - Getting Things Done, by David Allen, Things is an indispensable app, My third most used app next to Mail and Safari. It also has great support for syncing with the iPhone so you can take your lists on the road and store your tasks or ideas wherever you are.