org-mode for Emacs. Version 4.67 is included in Emacs 22.1, the latest is 5.07. Official manual. Screencast demo by Scott Jaderholm. Using Org-Mode as a Day Planner by John Wiegley. More on EmacsWiki.

This thing is seriously amazing. It's a super-outliner, it's a productivity manager, it's a notetaker, it's a lightweight structured text system, it's a lightweight spreadsheet, and it's all of those things without feeling bloated or overloaded.