Trip Advisor

About:

Trip Advisor has over 7 million accommodations, airlines, attractions and restaurants, providing travelers with the wisdom of the crowds to help them decide where to stay, how to fly, what to do and where to eat. TripAdvisor also compares prices from more than 200 booking sites so travelers can find the lowest price that's right for them.

Integration: 

Trip Advisor is a two-way integration, meaning you can synchronize your calendars with other parties to reflect the same availability using iCal.

NB: iCal based integrations are not real-time and carry delay in availability updates from showing.