If you're looking for a small, great-sounding, non-Apple music playing device, two of the most frequently recommended players out there have been the Sansa Fuze and Cowon D2/D2+. With the Fuze just getting bumped up to the Fuze+, it's only fair that Cowon release an update of their own. Although it was only last year that the D2+ was released, a certification filing on the 28th of September shows that there truly is a D3 coming, and hopefully soon.
The popular, open-source media player firmware Rockbox has recently been made available for Android. Rather than run as an operating system, Rockbox operates as a standalone application that you can install as usual with an APK. Development is still on-going, but Rockbox dev kugel has a few pre-compiled APKs hosted for you to try out if you’re interested.
These builds are unofficial and are purely for demonstrative purposes, but they seem to work pretty nicely.