Last year, I have developed a JS library SM.js (https://github.com/slaypni/SM-15) which is based on the algorithm used in the latest SuperMemo.

I have also developed a web service (http://slaypni.github.io/flashward/) for memorizing Foreign words using the library. Through the experience of using it by myself, I felt that spaced repetition helped me to remember many words for learning English.

I am feeling that most (free) SRS are using relatively simple algorithms (for example, Anki is based on SM-2 which is developed in the late1980s). Although I am not sure SM-15 is better enough, there could be some room to improve it. It is nice if the library was helpful for some developers who create new SRS.