Download Manager for Mac OS X
There are several choices :


Folx : 



  Folx is a free download manager for Mac OS X with a true Mac-style
  interface. It offers convenient downloads managing, flexible settings,
  etc. Folx has a unique system of sorting and keeping the downloaded
  content.





iGetter: 



  iGetter is a full featured download manager and accelerator. With
  iGetter you can get the most out of your Internet connection be it
  dial-up, wireless, cable, DSL, T1 and other high end broadband
  connections. iGetter can greatly improve the speed of your downloads
  using segmented (accelerated) downloading.





Leech (paid one) : clean UI.



  Every modern browser has its own download functionality, so why should
  you use a separate application for handling downloads in the first
  place? Because Leech works better than any built-in download manager.





jDownloader 2



  JDownloader is a free, open-source download management tool with a
  huge community of developers that makes downloading as easy and fast
  as it should be. Users can start, stop or pause downloads, set
  bandwith limitations, auto-extract archives and much more. It's an
  easy-to-extend framework that can save hours of your valuable time
  every day!



 - Fat Pipe: (free)


  Most servers and internet service providers limit the speed a single
  connection can have. We can bypass these limits by splitting a
  download task up into many smaller chunks. This guarantees that you
  will fully saturate your internet connection.


http://fatpipeapp.com/media/demo.mp4