Open-source web application Thingler creates real-time to-do lists so you and one or more collaborator can work on building and crossing items off a list without reloading the page.
Thingler is a very simple app. You visit the homepage, and it automatically loads a new, unique URL you can share with your collaborators. (It's very similar to the real-time, collaborative note-taking tool Etherpad in that respect.) When you add an item, it instantaneously shows up on your collaborators screen — and likewise when you or she removes an item, checks an item as complete or re-orders the list. There's not much more to it beyond that, but if you're working on putting together a list with someone else, it's a nice place to start.