The Rio Receiver was a home stereo device for playing MP3 files stored on your computer's hard drive over an Ethernet or HomePNA network.
With a design derived from the existing Linux-based Empeg Car, it became popular among the Linux hacking community.
The user interface was a 128x64 pixel monochrome LCD with an EL backlight, a rotary control with a push button, several buttons and IR remote control.
The second kernel then mounted a root filesystem over NFS containing a small set of standard POSIX tools and an application for selecting and playing music over the network, which was served using HTTP by the Audio Receiver Manager software running on a Windows PC.
Although the music player and the Audio Receiver Manager and Broadcom HomePNA kernel driver module were proprietary software, the kernel and other tools were open source.