Rockbox is an open source firmware replacement for a growing number of digital audio players. It has been in development since 2001 and receives new features, tweaks and fixes every day to provide you with the best possible music listening experience. Rockbox aims to be considerably more functional and efficient than your device's stock firmware while remaining easy to use and customizable. It is a goal to not only offer a wide range of functionality, but where possible make sure that this functionality is presented in a consistent manner that is easy to learn and use. It is written by users, for users.
Another top priority of Rockbox is audio playback quality. Rockbox, for most models, includes a wider range of sound settings than that device's original firmware. A lot of work has been put into making Rockbox sound the best it can, and improvements are constantly being made. All models have access to a large number of plugins, including many games, applications, and graphical demos.
Read on here.....However, it is not stable 'yet' for Sansa Clip Zip.
http://www.google.com.au/url?sa=t&rct=j&q=What+is+rockbox&source=web&cd=2&sqi=2&ved=0CDIQFjAB&url=http%3A%2F%2Fwww.rockbox.org%2Fwiki%2FWhyRockbox&ei=gTBXT7GrD_CZiAfqrJS9DQ&usg=AFQjCNGtqvKa51xcTK4azaRDCQ3q4eJeZA&sig2=Rl-TMqvevUM-soWtc42SvA
Rockbox web site link......
http://www.google.com.au/url?sa=t&rct=j&q=rockbox&source=web&cd=1&ved=0CDoQFjAA&url=http%3A%2F%2Fwww.rockbox.org%2F&ei=ozFXT7elBKqOiAeOqcylDQ&usg=AFQjCNFtCPEUyZtNPgyfkuQyrPyqgUOwfg&sig2=u_79DG4UMAjXEZH9aTAiMQ