Another comment has already mentioned Taskforce; Mail Pilot (http://www.mailpilot.co/), which was another Kickstarter project, also does something similar.
I really want something that does this, but as a native desktop app and a mobile app, and syncs to a web app so I can access it from elsewhere. And a pony.
MailPilot is also about to open up a beta today in about 4 hours, so not sure it's worth investing in another kickstarter to make a similar project. I want to play around with MailPilot to see if I like the metaphor of my inbox as a task list.
I really want something that does this, but as a native desktop app and a mobile app, and syncs to a web app so I can access it from elsewhere. And a pony.