Easy!Appointments is a highly customizable web application that allows your customers to book appointments with you via the web. Moreover, it provides the ability to sync your data with Google Calendar so you can use them with other services. It is an open source project and you can download and install it even for commercial use. Easy!Appointments will run smoothly with your existing website, because it can be installed in a single folder of the server and of course, both sites can share the same database.
This blog will contain posts about the project usage and development. You can find more information on the following links.