WhenWorks eliminates the hassle of scheduling meetings by allowing clients and colleagues to automatically book time with you online, only when you're free and only during the time slots you've defined.
Looks like a reasonable competitor to Calendly — similar functionality with the difference being that WhenWorks has its own native iOS app.
There are many competitive services in this space. What they all have in common is that they are purely web-based solutions. What makes WhenWorks unique is that it is a mobile app that integrates directly with the Calendar app on your iOS device, is far easier to configure and use, more secure, and always with you when you need it.