[jerrell]

I'm surprised not to see anybody mention Nozbe.

It's been around quite a while as a web app, and does a great job of balancing simplicity with functionality and features. The web interface is excellent for just letting you get on with what you want to do - i.e. things work just as you'd expect, and there are no frustrating UI punishments for doing things 'the wrong way'. The iPhone counterpart has recently had a major overhaul and is now just as slick and easy to use.

It's pretty hacker friendly, too - for example, you can export all your task info to a plaintext file, edit it however you like, and later re-import. There's a simple syntax for adding new tasks with metadata by email or Twitter.

I'd highly recommend giving it a try.