I’m not aware of other services that are similar to Zapier, but I’m sure there are some out there. You can make requests directly to the Discourse API without using a third party service. The main requirement for doing this is that you have an application to make the requests from. This can be something a simple as a WordPress site.
A good resource on Meta for getting started with the API is Reverse engineer the Discourse API. You can test API requests with CURL from your computer’s terminal, or with a service like Postman.