Transmission is a BitTorrent client which features a variety of user interfaces on top of a cross-platform back-end.
Syncthing is a free, open-source peer-to-peer file synchronization application. It can sync files between devices on a local network, or between remote devices over the Internet.
Mainsail is a lightweight & responsive web interface for the Klipper 3D printer firmware. It communicates with the Moonraker (Klipper-API) from Arksine.
Miniflux is a lightweight RSS feed reader app.