Simple, resource-friendly, cross-device synchronization
None so far
If you're a minimalistic approach lover then you should definitely check Wunderlist. It's doesn't eat much resources if you keep it open all the time. Lists of tasks, starring tasks is the best simple and productive way of tasks' prioritization. It also has a great feature of adding a task or searching for a task without going to the main window. Just use a keyboard shortcut for this.