Platform: Windows, Linux and more
- Syncs with the Wunderlist webapp and the Wunderlist app for Windows, Mac, Android, iPhone and BlackBerry
- Share lists with friends via Facebook, Twitter and other apps to collaborate with (or motivate) each other on certain tasks
- Add notes to tasks
- Organise tasks through a drag-and-drop interface
- Assign deadlines to tasks
- Filter tasks by status or date
- Star important tasks
- Add tasks via email
- Choose from several themes to personalise Wunderlist
If you're on a computer that doesn't have Wunderlist installed, you can just as easily check the web app and manage your tasks from there. Plus, like most other Linux apps, it's completely free. There's also something to be said for its fantastic interface, something you don't see as often on the Linux platform.
If you're more a fan of the GTD method of managing your tasks, GeeTeeDee is a great cross-platform client available on Linux. Its interface is once again very simple, but its got a lot of more advanced features that Wunderlist doesn't have, though they can just get in the way for most people. If you want an even more complicated GTD app, you might check out ThinkingRock, which is a bit more on the project management side.
If you're OK with using a web app, you have even more options. Remember the Milk (which, again, syncs with the aforementioned Tasque) has long been a favourite of users everywhere. It's available on multiple mobile platforms, and has a lot of organisational features. Many people like Google Tasks for its integration with Gmail and Google Calendar, though its a bit feature-poor for those that want more advanced management. It also isn't the most intuitive to use.
If you want a more advanced to-do app that integrates with Google Calendar and Gmail, Producteev is a better option, which has some nice advanced management, helpful collaborative features, and an app for the Mac (with a Windows version on the way). Lastly, Toodledo is a very powerful manager that's a bit hard to get started with at first, but lets you heavily customise how you manage your tasks, so you don't need to be stuck with any one method pushed by an app.