Cal.com

Cal.com is an open-source tool for online scheduling of meetings and appointments – essentially an open alternative to Calendly. It started as a public project with transparent code on GitHub and is backed by both a community and a company that also offers it as a ready-made hosted service. It suits anyone who books meetings with clients: consultants, coaches, hair salons, physiotherapists, freelancers, and small teams that want their bookings under control and compliant with European rules.

It works simply: connect your calendar (Google, Outlook, and others), set when you're available and how long meetings should be, and you get your own booking link. Send it to a client or put it on your website. The client sees only free slots, picks one, fills in a few details, and the booking is automatically written into both calendars. Cal.com prevents conflicts, so no one can book a time when you're already busy.