There are a few things that I think are really interesting about Google's Calendar API. First, it immediately becomes another mashups medium (like maps). Already there are tons of public calendars, from religious calendars, to search conferences to Chamber music festivals online. (I anticipate someone will find a way to mashup maps and the Google Calendar :) .)

To access any of these public calendars, one just searches in the "search public calendars" field (e.g., "chamber music" or "Jewish holidays") to find them. With a click of the "add to calendar" button, those events/schedules are quickly plotted on your calendar. And you can turn them on and off with a check box; it's very easy.

Putting aside the calendar sharing possibilities among professional groups, family members and friends that are themselves very interesting, these new public calendars become another valuable promotional or informational tool (to the extent Google Calendar gains adoption) for entities and publishers. So far, I'm using Calendar and generally really like it. (It needs to be integrated with the Google Personalized Homepage and probably soon will be.)

With all these events and schedules already online, Google Calendar becomes a Local Search tool as well. Who's playing at my favorite local club? What are the upcoming exhibits at the San Francisco Museum of Modern Art? What's the schedule for the Philadelphia Symphony Orchestra? What author is speaking at the local bookstore on Sunday? There are already hundreds of these calendars it appears and a large percentage of them have a local dimension.