Download Navi is a free and open-source download manager.

Features
• Free and Open Source software (FOSS)
• Material Design
• Multiple themes (Light, Dark, Black)
• Pause and resume downloads
• Browsers support
• Simultaneous downloads (default 3)
• Multi part downloading (max 16)
• Download files in background
• Unmetered connections only support (ex. Wi-Fi)
• Roaming controls
• User agent controls
• Calculate hash value (MD5 & SHA-256)
• Power Management and battery control
• Various automatic actions after finishing download

Source code
https://github.com/TachibanaGeneralLaboratories/download-navi