I saw that you have already some right here on this site, and the others gave recommendations for more.
You could also put testimonials on your website where the issue of registration does not exist. I recently had a wonderful comment from a participants in my class, and I asked her whether I could put this on my website. She was happy to agree to it. There you can ask the person how they want their name to appear. Not everybody wants to have a ‘web presence’.
I found most of my clients very happy to give testimonials on Fitness Connect. A few did not because IDEA insists they register and they were not happy about that. I did assure them they would not be spammed, but they balked at the registration. One of those same clients did write me a lovely recommendation for an award a while back, so for her the stumbling block was the computer registration, not the recommendation.
I always make sure to tell them it is optional, and thank them for their business either way.