The existing products in this space include ngrok, HookDeck, Convoy and others. They are all great, but they have some caveats. For one, ngrok doesn't provide stable URLs for webhook development unless you fork over money for a paid plan. For an indie/hobbyist developer just trying to test their app, this can be a non-starter. Meanwhile, HookDeck and Convoy are full-fledged webhook relays which can be complicated to use.
Trout is a simpler way to do webhook development. You can create sources and plug them into your external services e.g. Stripe. Then, you can forward these events anywhere. Trout also comes with a CLI you can install and use to listen to events on any source. This makes developing on localhost easy.
All feedback is welcome!