Webhooks are fantastic. We use them here at Shopify to notify API users of all sorts of important events. Order creation, product modification, and even app uninstallation all cause webhooks to be fired. They're a really neat way to avoid the problem of polling, which is annoying for app developers and API providers alike.
The trouble with Webhooks is that you need a publicly visible URL to handle them. Unlike client-side redirects, webhooks originate directly from the server. This means that you can't use localhost as an endpoint in your testing environment as the API server would effectively be calling itself. Bummer.
Fortunately, there are a couple of tools that make working with webhooks during development much easier. Let me introduce you to PostCatcher and LocalTunnel.
PostCatcher
PostCatcher is a brand new webapp that was created as an entry for last week's Node Knockout. Shopify's own Steven Soroka and Nick Small are on the judging panel this year, and this app caught their eye.
The app generates a unique url that you can use as a webhook endpoint and displays any POST requests sent to it for you to examine. As you might expect from a js contest, the ui is extremely slick and renders all the requests in real-time as they come in. This is really useful in the early stages of developing an app as you can see the general shape and structure of any webhooks you need without writing a single line of code. On the flip side, API developers can use it to test their own service in a real-world environment.
The thing I really like about PostCatcher over similar apps like PostBin is that I can sign in using github and keep track of all the catchers I've created. No more copy/pasting urls to a text file to avloid losing them. Hooray!
LocalTunnel
LocalTunnel is a Ruby gem + webapp sponsored by Twilio that allows you to expose a given port on your local machine to the world through a url on their site. Setup is really easy (provided you have ruby and rubygems installed) and once it's installed you just start it from the console with the port you want to forward and share the url it spits out.
From then on that url will point to your local machine so you can register the address as a webhook endpoint and get any incoming requests piped right to your machine. My previous solution was endless deployments to Heroku every time I made a small change to my code, which was a real pain in the arse. Compared to that, LocalTunnel was a godsend.
Alternatives
Whilst PostCatcher and LocalTunnel are currently my top choices for testing webhooks, they're by no means the only party in town. I've already mentioned PostBin, but LocalTunnel also has a contender in LocalNode (another Node KO entry). The latter boasts wider integration (you don't need ruby) as well as permanent url redirects but setup is more complicated as you have to add a static html file to your web server.
If there are other services, apps, or tricks that you use to test webhooks when developing apps, call them out in the comments! I'd love to hear what I've missed in this space.