Very lightweight, simple and feature-full download tool.
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Pro Small memory footprint
Due to aria2 being a CLI only tool it uses very little memory.
Pro Supports multiple download protocols
Has support for lots of download protocols including BitTorrent, Metalink, HTTP, HTTPS and FTP.
Pro Remote control over simple standartized RPC
XML or JSON-based RPCs can be used to control aria2 remotely with both APIs completely documented. Simplest client that uploads torrent to be downloaded is 3 Python lines long (no 3rd-party libraries, only stdlib).
aria2c --help=#rpc to see more.
Pro Third parties offer a clean web interface
Some web interfaces consist of pure HTML+JS, no server-side needed (except aria2c of course). It uses the same RPC that can work even over WebSockets.
Pro Free and open source
Con No native graphical user interface
While there is a web-based GUI, there is no native interface.
Con No HTTP/2 support
While the developer of aria2 is developing nghttp2 library, aria2 has no support for http2.