The Wunderlist to-do application (Free, Mac App Store) by 6Wunderkinder is an excellent free cross-platform task management tool. It's available for Windows and Linux, and there's even a Web app if you're gun-shy about downloading a standalone app. Wunderlist enables users to share tasks and lists to colleagues or friends, add to-do items via email, and organize tasks with a simple drag-and-drop interface.
What's perhaps most attractive about Wunderlist is that you can share tasks with others who don't use the to-do application; although you can send your recipient a sign-up invitation, the desktop version of the app will create a plain-text email packed with the essentials. And if you have the shiny new iPhone 4S, you can use Siri to dictate tasks to Wunderlist.