Zoom. A video meeting link gets generated and attached to an appointment automatically when Zoom is connected, no manual step for either the business or the client.
Google Calendar. Connected separately from the API, through Acuity's own account settings, this is the two-way sync covered on a given calendar: availability checked, bookings written.
Payment processors. Square, Stripe, and PayPal connect natively so a deposit or full payment can be collected right at booking, with the transaction tied to the appointment record.
Zapier. Acuity Scheduling has a Zapier app covering common single-step triggers, new appointment created, appointment canceled, enough for straightforward connections to a wide range of other tools without custom code.