Personally, I use Yahoo calendar to keep track of my "to do" list. However, there are a lot of limitations.
One of my friends swears by a free site called Remember the Milk. They will let you set up a calendar, but also do more advanced things like map directly to locations, and get reminders over cell phones, email, or instant messenger programs. It also works and directly synchronizes with Google, the iPhone, Blackberry devices, and Twitter.
If you are not all that organized, it might be worth trying out!