I use Aniseed, but it's worth noting that it's something like a "framework" on top of Fennel and Neovim. So I also want to shout out Hotpot, which is more of a simple plugin that allows you to put plain Fennel files in your runtime path, without any additional "framework" to learn.

That said, I think the framework that Aniseed introduces is really nice, and I think would be useful for writing general purpose programs outside of Neovim.