New York-based four-year-old startup ZocDoc is making big waves with its fast-growing app that allows users to search for doctors or dentists, read reviews, and schedule appointments instantly. Adding to heaps of media hype, the service just launched in Boston and last week started to roll out a new feature - waitlists that ping users when a booked-up doctor has an appointment open up.

Currently ZocDoc is available in 11 US cities and for some 40 different doctor specialties, and the company plans to expand its service, which is free for users, throughout the country over the next year.

While ZocDoc may be leading the charge in medical booking apps according to media reports, syncs with practice management programs for doctors, which ZocDoc does, but also offers online secure patient registration and referral.

For users around the globe, app developer Healthagen, which offers the iTriage mobile app, announced earlier this year that it will be adding appointment booking to its app; dates when users can expect to see the new feature rolled out have yet to be announced. The app, available in more than 80 countries, allows users to research medical symptoms, locate their nearest doctors or hospitals, and view hospital emergency room wait times.