Orchestra is a really fantastic to-do app for your iPhone that lets you collaborate with others even if they don't have a copy of the (free) app themselves. You can share anything you're working on, others can send you tasks requiring your help, and it's all remarkably simple to do.
Even if you don't plan to use Orchestra for collaborative purposes, you get a really great to-do app. It'll teach you how to use it with helpful tooltips the first time you launch, but it's very straightforward so chances are you could figure it out on your own without any help at all.
As you might expect, you start by adding tasks and assigning due dates, but you can add tasks by typing or speaking. I found the voice recognition to be remarkably accurate. I tried accents and even spoke with food in my mouth and it transcribed everything I said without a problem -- and quickly, too. (I imagine it makes mistakes from time-to-time, seeing as humans aren't even that good at speech recognition, but it understood me just fine.)
Once your tasks are in, you can swipe them for quick edits -- such as choosing new sorting options -- or use one of the many viewing options to look at only what you want to see at the moment. All of your changes are synced to the web so you can access them from any connected computer when you don't have your iPhone (or would simply prefer to work from a browser).
Then you have what makes Orchestra different from the other to-do apps: collaboration. You can easily collaborate with other Orchestra users to share tasks. It's almost like sending text messages to each other, but with a productivity element. If you want to share a task with someone who isn't using the app, however, you still can. So long as they have an email address you can loop them in on something you need to get done.
I'd initially hesitated to check out Orchestra because I was skeptical of yet another to-do app, but it's a step above the rest. Regardless of how you want to use it, it's fantastic and I highly recommend checking it out.
Orchestra To-Do (Free) [iTunes App Store]