I have been thinking about writing an outlook add-in that would do the whole free and busy thing (appointment calendaring) with a web application/service. I have wanted this since netfolders, because I want people to be able to see my calendar but not the details; so people can schedule when I am available. It is hard for me to believe that no company has written this. I think it would be really useful (maybe its just me)... Anyway Microsoft talks about howto write an Outlook add-in using .Net here:

http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dnout2k2/html/odc_oladdinvbnet.asp