I'm about to write my own .ashx handler to receive hCard data and return a properly-formatted VCF file and - when I get around to it - to handle hCalendar events as well. I know the Microsoft Oomph project does something very similar (in fact, I plan to base much of my work off that service).
Are there a range of open services available that do the same thing? Is it worth doing my own (if not purely for the learning experience)?