Many developers expose their apps' functionality through custom URL schemes. The built-in Maps.app is a good example, allowing other apps to open up the app displaying a specific location. handleOpenURL, maintained by @Magnatron, is a directory of custom URL schemes. You can look up the URL schemes supported by other apps as well as register those supported by your apps.

Your feedback is valuable: Do you want more nuggets like this?   Yes   or   No