Cal.com - Cal.com (formerly Calendso) is the open source Calendly alternative.
Pick - The simplest way to schedule a time to meet
Cron - Cron Calendar.
Meekan - Connecting the world's calendars
Calendly - Say goodbye to phone and email tag for finding the perfect meeting time with Calendly. It's 100% free, super easy to use and you'll love our customer service.
LettuceMeet - The easiest way to schedule group meetings.