For the past few years I've been using Outlook (at work) to keep track of my calendar. I find that if I don't put an appointment into Outlook as soon as I schedule it, I'm highly likely to forget it.
However, I've been slowly growing frustrated with Outlook. I can log into my work e-mail from off campus and view my calendar remotely, but the calendar is extremely cumbersome when used via the web interface. Since I track both work and personal events on my calendar, it's annoying that the best place to access my calendar is at work. One theoretical benefit of hosting my calendar at work is that it's possible for me to share it with colleagues (e.g., to make scheduling meetings easier), but no-one has ever taken advantage of that ability.
Enter Google Calendar; it was just announced to the public a few days ago, and I already like it. It's got the classic Google intuitive interface, has multiple options to send reminders about upcoming appointments, lets me specify which other users can view (and edit) my calendar (so I can share it with my SO), and has a built-in invitation manager to handle RSVPs and the like. It's little things like being able to change the length of an appointment with one click and drag (rather than having to open up the full event window), getting e-mailed a daily agenda every day at 5am, and being able to display multiple people's calendars simultaneously, that make me like this application. And, of course, it's free to use; if you already have a Google account you don't need to sign up for anything new.
Because I only had a few dozen events upcoming in Outlook, switching was relatively easy - I just re-typed everything in from scratch (which was quick thanks to Google's great interface). However, for those with busier calendars, Google has the ability to import calendars from other programs (including Outlook).
My sole gripe is that the notification feature needs a little work. Right now the popup notification only works when I have a web browser open to the calendar page; it'd be nice if it interacted with the Google Talk application so I'd get reminders even if I didn't have a web browser open. Making things a bit worse, the popup notification also has a habit of grabbing the focus of the web browser, switching me away from whatever page I'm working on. The program also doesn't have any options to snooze an alert, a feature I use regularly in Outlook (e.g., if I want to be reminded the day before a meeting, and also five minutes before it). However, I expect that they'll improve this over time, and everything else works great.
If you want to learn more, this blog has a good review, though I'd suggest just trying it out. It's very easy to use.