I've been using a beta of TaskPaper from my favourite micro-ISV for a while now and it is amazing how much it hits the 80-20 point of what I want in a todo list manager.

TaskPaper manages to support projects, tagging, filtering and archiving of done tasks all with a human-readable plain text markup format.

It's just gone 1.0 and I thoroughly recommend checking it out if you run Mac OS X.

UPDATE (2007-10-29): After writing this, Jesse Grosjean offered a free license to people who wrote reviews on their blog. I decided not to take up this offer and mention that fact here in the interest of full disclosure.