You were able to install Rockbox (an aftermarket ROM for Apple, Sansa, and a few other mp3 players). Was quite good, had better codec support (played OGG files!

You were able to install Rockbox (an aftermarket ROM for Apple, Sansa, and a few other mp3 players). Was quite good, had better codec support (played OGG files!) and you could write apps [1] for it. [1]. Don't know if you could call them apps, as they needed to be rebuilt with the OS (don't remember if they were pluggable modules or you literally had to rebuild the OS if you wanted to write an App, but it came out of the box with a few apps and games, and you could write your own in C. Their API was weaker than Android's, but to me more enjoyable.)