Fantastical is a fantastic tool for the Mac that I've been using for quite a while (here's our comparison of Fantastical and Today). It lives unobtrusively in your Menu Bar and offers fast, natural language creation of calendar events. You can also use it to edit existing appointments, browse what you've got scheduled and sync with iCal, BusyCal, Entourage, or Outlook.
Version 1.2.2 prepares the app for Gatekeeper, allows for vertical resizing of the event list (Lion only) and adds notes to search results, which I appreciate. There's more, of course, and you'll find the full release notes here.
Fantastical requires Mac OS X 10.6+ and is available for $19.99. This is a free update for existing users.