Zoho Plug-In for Microsoft Office lets you: Create, edit and save their documents & spreadsheets directly to Zoho Writer/Sheet from within Microsoft Word/Excel. Alternatively, users can open and edit their Zoho documents & spreadsheets in Microsoft Word/Excel and save it back to their respective Zoho services (Writer/Sheet).

It blurs the line between web service and client-side application and creates a hybrid environment that lets you work with whatever tool is most convenient at the time(web-based Zoho Office or PC-based Microsoft Office) without having to sort out where the document lives. It has been around for a full year. I only got around to mentioning it now :-)