Mozilla Sunbird is a cross platform standalone calendar application from the Mozilla Foundation. Though in the "experimental stage" it is pretty nice, and uses the same format as iCal on your Mac, so you can share and publish you calendar cross platform.
I've been looking for a decent Calendar Application for Windows since I ditched Outlook for Thunderbird a year or two ago. I installed Mozilla Calendar as a firefox extension, but I didn't like having to use Firefox to get to my calendar. The standalone version is what I was looking for! This is also nice because I can sync my work calendar with the calendar on my PowerBook.