ousia VS owlkettle

Ones that have not been mentioned so far:

nlvm is an unofficial LLVM backend: https://github.com/arnetheduck/nlvm

npeg lets you write PEGs inline in almost normal PEG notation: https://github.com/zevv/npeg

futhark provides for much more automatic C interop: https://github.com/PMunch/futhark

nimpy allows calling Python code from Nim and vice versa: https://github.com/yglukhov/nimpy

questionable provides a lot of syntax sugar surrounding Option/Result types: https://github.com/codex-storage/questionable

ratel is a framework for embedded programming: https://github.com/PMunch/ratel

cps allows arbitrary procedure rewriting to continuation passing style: https://github.com/nim-works/cps

chronos is an alternative async/await backend: https://github.com/status-im/nim-chronos

zero-functional fixes some inefficiencies when chaining list operations: https://github.com/zero-functional/zero-functional

owlkettle is a declarative macro-oriented library for GTK: https://github.com/can-lehmann/owlkettle

A longer list can be found at https://github.com/ringabout/awesome-nim.

Although for a GUI (if I even made one, rather than just a fixed/hardcoded setup) I'd probably use Owlkettle(though there are some things I don't like about it, see closed issue 16).

A similar library in nim is https://github.com/can-lehmann/owlkettle