Cal.com - Cal.com (formerly Calendso) is the open source Calendly alternative.
Geekbot - Discover how to organise asynchronous stand up meetings in Slack and keep your team synced using Geekbot. Start your free trial today!
WhenWorks - Mobile appointment scheduling
Chili Piper - Chili Piper is an intelligent calendar for Sales teams, to book their own meetings or set appointments for other teams.
Inspora - virtual chat-stylist for fashion advice.
Standuply - Run daily standup meetings and track your metrics in Slack