"Mood rings" change colour based on the surface temperature of your skin, so I use mine as a simple, inexpensive biofeedback device. Since my fingers and toes tend to get very cold when I'm nervous, anxious, or my breathing is shallow, the change in the ring's colour often alerts me before I'm aware of it myself, and I know to calm down. As someone who does a lot of performance and public speaking, it's a very good way to monitor stage fright! While not the most scientifically accurate device, it works for very simple self-monitoring, and it's certainly the most stylish.
It has a nice design and changes colours fairly accurately. Does not look like the picture though as it is oval and not round. A wider range of sizes would also be nice as I have very small fingers. Otherwise it is a good product.