mruby[1] fits that category too. It can be used with H2O[2] server as well. There was a discussion here a couple of months ago regarding some use cases[3].

[1]https://mruby.org

[2]https://h2o.examp1e.net/configure/mruby.html

[3]https://news.ycombinator.com/item?id=34937297