Aria2 is a headless (command-line based) multi-protocol downloader. It supports the downloading of HTTP/HTTPS, FTP, SFTP, BitTorrent and Metalink resources. Many frontends exist for Aria2, they are available either as standalone apps, or browser extensions.
Technically Aria2 can function by itself, but I don’t consider its command-line functionalities to be usable for most. Thanks to its frontend-backend separation architecture, Aria2 can easily be deployed on headless servers, such as a VPS, where you can remotely push download tasks to the backend server. Since the actual download takes place on on the backend server and not your frontend device, you don’t have to worry about leaving your devices on or preventing it from entering sleep/suspend states.