A recent Weblogs.asp.net post heralded the release of PayPal's new ASP.Net SDK. They also expose many of their features as web services.
A clever SharePoint developer might create a web part or series of web parts that implement the ASP.Net web controls in the PayPal ASP.Net SDK. Also, consuming the PayPal web services using the data view web part is fairly interesting. Some of the services don't make sense being consumed by the data view web part, but it ends up looking much more professional than the PayPal button.
There is an interesting article on MSDN entitled “Custom Calendar Providers for Outlook 2003” which contains a hidden gem: a way to integrate RSS with a Calendar view in Outlook 2003.
I should also point out that this opens up the possibility for folks to have Outlook calendar view integration, previously reserved only for SharePoint lists based on the event list template, now available to arbitrary SharePoint lists or even document libraries. Not to say that this wasn't possible before, but now it is neatly documented.
I have been working to try to dust off the idea that Ray Ozzie and others have mentioned about providing a similar syndication format for calendar information. I think this is a marvelous idea. This all pivots around one of the key tenets of successful knowledge management which is unanticipated repurposing of information. This has been the hallmark of blogs' success in that folks who post an idea can have that posting repurposed all over the web in ways they had never originally planned. Schedules (not personal free/busy information) stand to benefit in an equally pervasive way.
RSS can be misused to represent this information, and quite wonderfully integrated into Outlook as mentioned in the above article, but it is truly not well suited for this purpose. Ultimately, an xCalendar format needs to represent specifically calendar information. Imagine being able to get a XML xCalendar feed from Craig's List of garage sales in your area using a calendar aggregator, or area events from eVite aggregated with events from Meetup, etc.