I just registered for the Brunch and Learn Marketo event in Boston, and saw that the 'Add to Calendar' link gave me multiple options (See below image).
Does anyone know how they were able to do this? It looks like it isn't through Marketo, but figured I would ask anyway since we would like to test this out.
If you haven't seen it, check out my blog post https://nation.marketo.com/message/138542
I haven't built Yahoo! link support, would be simple enough if you really need that option.
In case you were thinking that was something Marketo generated, it most likely isn't. You'd be needing to write the HTML for the popup with the individual links to the calendar files. This also means you'd need to create the calendar files and save them to a server.
However, a webservice like the one Sanford linked would take away the manual task of creating calendar files, saving them to the server.