Wunderlist is a really awesome task list app I discovered this week. I feel like it has everything a task list app needs. It's free, has no ads and is cross-platform, which is really nice since it enables you to sync your tasks between all of your devices via your account. A particular task can have subtasks, a due date, a reminder and a note. You can also have different task lists.
The app also has smart lists for starred items, today's tasks or the upcoming week's tasks. You can also choose to display them all. They previously had a smart list for tomorrow's tasks, but oddly it has been removed in the new version. Fortunately, due to the tons of requests to add it back, they just announced it's planned.
Wunderlist currently has clients for Windows, Mac OS X, iOS and Android. There's also a web app available, that also has a mobile version, so you can access your tasks from anywhere. There is no Linux version yet, but it seems to be planned, according to what we can read on UserVoice. You can find a guide to install the old version on Ubuntu here.
Here are download links for the different platforms:
To create an account, head over to Wunderlist's website.