One of our favourite to-do lists/task manager apps for Android is Astrid. We love the deep feature set, the advanced tinkering and the easy to manage interface. Now it's on the iPhone. And even though the iPhone has a ton of apps like Astrid, Astrid is already one of the best.
It's a task manager. And yes, task managers are snooze inducing and rip your eyeballs out boring but they have a place in our go go go world. Using daily planners or calendars just isn't enough anymore with all the goings on. Astrid on the iPhone is just like the Android app—even down to the looks—which means you can create tasks, priority levels, integrate with Google Calendar, sync with Google Tasks and set up tags, alerts, periodic reminders and more. There's also notifications and you can even share tasks with other Android users across platforms (platforms being iOS and Android).
Because as full featured as Astrid is, it doesn't overwhelm you with a clunky user interface. It's the all powerful, task managing assistant that you don't have to learn how to use. The app uses voice recognition to turn your speech into tasks and the periodic reminders are especially good at, well, reminding you what you need to get done. Probably the best feature is sharing tasks, if you're collaborating with someone at work or doing something with the wifey or planning a party, Astrid can sync what needs to be done on your to-do lists, complete with due dates and deadlines, across phones. It's like having one personal assistant for multiple people.
Astrid To-Do is available on the iPhone for free