Wunderlist is a pretty handy to-do list and task manager. It has a user-friendly interface and can be accessed on a wide variety of sites. Thanks to Wunderlist for Ninja Forms, you can integrate this service with your WordPress site. This script uses emails and actions to add tasks to Wunderlist automatically.
You can simply set this up to link actions to Wunderlist tasks in different lists. It supports Merge Tags. You can find out more about it here.