Rockbox is an open-source project that replaces the firmware on many dedicated MP3 player devices. It's still under active development. Their most recent releas

Rockbox is an open-source project that replaces the firmware on many dedicated MP3 player devices. It's still under active development. Their most recent releases improved power usage to the point where my 6 year old Sansa Clip+ now has better playtime than it did on stock firmware when the battery was new. It also produces objectively better sound reproduction, and can handle FLAC which the original firmware could not.