We've used both LivePerson and LiveResponse (http://www.kayako.com http://www.kayako.com). We preferred LiveResponse as it is incorporated with their help desk

We've used both LivePerson and LiveResponse (http://www.kayako.com http://www.kayako.com). We preferred LiveResponse as it is incorporated with their help desk software which is fairly robust. Both were easy to use and install though. If you are looking for something simple then I would recommend LivePerson.