Fennel is really cool, and the fact that it builds on Lua—an already-popular language—is a huge win; Fennel gets access to the entire Lua ecosystem. Importantly, Lua is pretty easy to embed in native applications. I haven’t found a Lisp or Scheme as easily-embeddable as Lua(+Fennel). It’s also a good drop-in replacement for Lua. I write my Hammerspoon config in Fennel, and hopefully my Neovim config soon as well.
There's also Spacehammer[1] which is all written in Fennel. Having to write Lua would have kept me from Hammerspoon.