I recently noticed that Google has started offering an online calendar that is similar to the one available in Microsoft Outlook.  From a first glance, it seems like a decent calendar application. 

They have implemented a lot of features similar to Outlook such as the ability to share your calendar with others.  You can delegate other people to have permission to edit your calendar.  It also provides the ability to schedule events, send invitations, and manage RSVPs.  Invitees can also post comments that are visible to other invitees  (sounds very eInvite-ish to me).  Once the event is configured, you even can add a button on your site that will provide event reminders.  Google has even created their own calendar buttons that can be used such as the one shown here.

They also have an import feature that can be used to upload an existing Apple iCal or MS Outlook calendar.  However, there is no export feature.  In other words, they would be happy to let you discontinue using other applications and load your data into their calendar, but don't expect an easy way to get your stuff back out if you don't like using it.

I will probably play around with the calendar, but I expect to continue using Microsoft Outlook for personal stuff.  It suits my needs and is right there with my email.  However, if you are looking for a freebie calendar that has quite a few features, it is definitely worth checking out.  I could see it being particularly useful for community-style sites that want to maintain a calendar of events for all users.