+1 for aria2! It does one job and does it very well.

Want to add a UI? Take a look at WebUI-Aria2: https://github.com/ziahamza/webui-aria2

It is also very easy to integrate with othersoftware via the RPC interface https://aria2.github.io/manual/en/html/aria2c.html?highlight.... You can find wrappers in some languages e.g. the JS bindings: https://github.com/sonnyp/aria2.js