Windows and Mac OS X: Wunderlist manages your tasks with a few focused features for organization, all within a beautiful interface and at no cost.
There are not many features to speak of in Wunderlist, but its minimalist focus is what makes it work so nicely. Aside from being really pretty, the interface is straightforward and easy to use. You can create lists, add tasks, decide when they're due, and search and sort through everything you add. All your tasks and lists are synchronized with your wunderlist account, so you can install wunderlist on as many Macs and PCs as you've got and keep everything in sync. If you're not online, Wunderlist will work just fine and will sync up next time you're connected.
Wunderlist is available right now for Windows and Mac OS X and is completely free. Mobile versions are currently in development.