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.
There may be broken links to media on this page. I'm still in the process of migrating them over.