If you are into David Allens Getting Things Done productivity system then you have a good few options on the Mac for software tools designed specifially for the task.

My favourite is iGTD by Bartek Bargiel. I use this all time and is my favourite by far. It syncs with iCal and also works with iSync, supports tags, links and address book integration.
Next thing I would love to see would be some team based features such as support for syncing with Basecamp and sharing tasks with other users.

Omnifocus from Omnigroup is looking good too, but is still in development as of this post.
They posted a video recently showing how it will work (Quicktime).

Other options I have used are:

Midnight Inbox
Actiontastic
Ghost Action

There is a useful Wikipedia entry here covering GTD software across all platforms.