Google Calendar is great...syncs across many devices...sometimes I forget and get so impressed. If set up right it will take any updates you make, put them on the cloud, then send them out to devices. So your calendar on your iPhone, iPad and Mac will are be the same. (and it work well with my phone OSes(I've used Symbian and Android without issue,I'm sure it works well in Windows.)
And you info is always on the web, if you find yourself without any of your own devices.
(Also it has a nice contact and task feature, plus all of these can be made into Web apps on the iPhone, they have to load, but google ends of making them feel like an app more then just a web page.