Anxiety
Quite possibly the coolest to-do list ever. A translucent window floats above your work, allowing you to add to-do items. The items automatically sync with your lists in Mail and iCal. Leopard only.
Do It
Manage categorized to-do lists with the option of linking each item to a file on your computer, a URL or a contact from your address book, which can be opened directly from the Do It window. Items can be set as reminders in iCal.
Remember
From the makers of Caffeine, a simple list of tasks that allows you to add folders. Focus feature shows items in one folder at a time. Can be a standalone window or a menu bar item.
SimpleTask
To-do list lives in menu bar, dock or both. Tasks are either pending or complete, in order of when they were created — or move them where you like. Searches can be filtered by colour labels. Optionally use the keyboard for all functions.