You can go and tryout mindbody. http://www.mindbodyonline.com/
Also a platform that’s being used very successfully (and very cost-effectively) by numerous trainers, teams, and gyms is called LogsItAll. It’s easy to use, flexible, scalable, customizable, and the company’s founder, Bill Patton, is an athlete, website is, www.LogsItAll.com, under the “For Trainers” tab
MYZONE is another affordable way to engage with and retain your members/clients. www.myzone.org is the website
Sorry I am not sure how long either of these have been around but you can ask that question.