Download managers speed up and organize large downloads by splitting files into parallel connections, resuming interrupted transfers and handling BitTorrent or video sites. Browsers cope with ordinary files, so these apps matter mainly for big files, unreliable connections, torrents and saving online video.
Motrix is a free, open source manager for HTTP, FTP, BitTorrent and magnet links, built on aria2. qBittorrent and Transmission are open source BitTorrent clients, with Transmission the lighter of the two. youtube-dl and Downie save video from websites, and JDownloader handles file hosting services. Only download content you have the right to use.