I think Zapier and the likes fill this need pretty well. I wonder if there's still a niche for more lower-level hosted service for plumbing webhooks though. It might be a rather small niche unless you figure out a way to offer better value than those guys.

One thing that definitely makes it hard is the chicken&egg situation. I can't imagine many people paying for a service (or even using it for free), unless it has a solid reputation and is highly available. Otherwise, they'll either host something on their own, or use one of the bigger players.