I have created two just recently
1. Using Twilio to send SMS Arabic formatted payloads to leads in the Middle East.
2. I just made a web hook to call a custom SF APEX webservice to create person accounts. This was used to bypass the lead conversion requirement as the customer didn't use leads and was not interested in have any data in the SF lead object. Initially, I was hoping that Marketo could have done the whole process directly to SF but due to Marketo's inability to parse the return due to SOAP namespaces, I had to write an additional webservice as an intermediary with a JSON response that could be processed.
3. In the very near future, I am going to have Marketo process SF custom object data via the SF REST API
Cheers,
Eric