Having been in Marketo Enterprise consulting for about 2 years I've seen a fair few cool Webhook solutions.. here are a few of my favorites that may give you some inspiration
1. Calling a service that generates a unique software download URL
One of our clients wanted to generate a UNIQUE URL for a software download when a Lead filled out a form (and as a bonus, they became known marketable leads when they requested it). A third party takes in a unique super secret ID and generates that unique URL.
As you can see below, the jSON coming back is mapped into an attribute on the lead. We can then trigger a campaign to the user and send them their trial.
2. Sending SMS messages with Twilio
This one was one of mine, and I documented it right here on the Marketo Whisperer blog recently (Sending an SMS using Twilio)
3. Sending push notifications with Urban Airship from a mobile app (before there was Marketo Mobile Engagement)
Before there was Marketo Mobile Engagement, which you should absolutely check out immediately, there were other providers of simple Server Side Push messaging such as Urban Airship.
This one is really powerful because it also leverages other parts of Marketo like Custom Activities. You can imagine how cool it is to sign up for a mobile app and receive push notifications along with other marketing content. We can send reminders, contact other people and even do deep linking back into the Mobile app.
Here you'll notice that we get back some interesting tracking information from UA, which we could then report on using the UA Reporting API.