Linux: Want a to-do and calendar manager that puts very little fluff between you and your important stuff? Calcurse is a clever text-based task and scheduling app that's rich in features but light on interface doo-dads.
Based on the Curses interface system, Calcurse uses a pure terminal-text interface to show what's coming up in your schedule. It moves very fast, as you might guess, and can be customised to the hilt. Set Calcurse up to email you when events are coming up, import or export iCal feeds, attach notes to appointments and edit them with your chosen editor—the whole system is up for whatever you'd like, as long as you don't mind digging into some configuration files.
Calcurse is a free download for Unix-based systems of all stripes. Source code and pre-built packages are available at the link.