I think a) is the most unique - there are already quite a few companies going after b) (FengOffice, Zoho Project, etc), and c) seems like it would be more diffi

I think a) is the most unique - there are already quite a few companies going after b) (FengOffice, Zoho Project, etc), and c) seems like it would be more difficult to monetize. It also might be easier to develop and market a Facebook App, depending on your skill-set.