Mac OS X only: Most good calendars apps have some sort of natural language quick-add feature so you can type "dinner tomorrow w/jerry at 8" and expect semi-accurate results. QuickCal is like that, but much, much better. QuickCal integrates with iCal, so each time you enter a new event, it'll show up in its respective spot in iCal (and, if you're syncing Google Calendar with iCal, it'll go there, too). Probably the best thing about QuickCal is that it analyses the appointment as you type, so you get a preview of the accuracy of the natural language interpretation before you create the appointment. (I've run into a lot of natural language failures when creating events using other tools, but I never knew it until after the bum event hit my calendar.) Its translation—in my tests, at least—have been dead on, but it's nice to verify the outcome of your event before you create it.
The next best feature QuickCal brings to the table is smart reminders, which it creates based on how far into the future your appointment is. By default, that means that all events are created with a reminder 15 minutes prior to the event. If the event you've created is more than a week away, QuickCal will also set a reminder one day in advance. If it's more than a month away, you'll also get notice one week ahead of time. I love this feature. The only downside to QuickCal: It's a Dashboard widget. Your feelings may differ regarding the Dashboard, but I'd much rather have a menu bar application that I can invoke with a keyboard shortcut and start typing into instantly. For the functionality, however, I'll gladly move to my Dashboard when necessary. QuickCal is a free download, Mac OS X only. Thanks Pat!