Fennel is remarkable for a few reasons. I'm a big fan of Phil Hagelberg's work in general, and Fennel is no different. It's a tiny library that allows lisp programming in Lua, and when coupled with LuaJIT, provides a tiny, fast, portable scripting environment.
In particular, I'm impressed by Fennel because it is focused on limiting runtime cost vs. Lua and permits compilation of Fennel to Lua via
--compile, which is fantastic in retaining the slim dependency tree I love about Lua scripts.