This app isn't free but the trial version is. PowerAmp, its by far the BEST music app out there offering the most options (EQ, Headphone Control, Custom Themes, etc) and the best GUI I have seen!
And no I am not being paid by PowerAmp to write this, the app is just that good!
Other Free Apps / Games I like Include:
1. Android Central (of course!)
2. Avast
3. Burger
4. Dead Trigger
5. Google Chrome
6. Catch Notes (this app has saved so much time and frustration when it comes to trying to remember what I need to get at the store!)
7. Plague Inc. (Kinda a twisted game but very entertaining)
8. Ulysse Speedometer (At night and not so bright days this app can be used to make a cool HUD while driving if your car doesn't already have that feature)
9. Wind- Up Knight
10. Airdroid (the BEST app out there for controlling your device through WiFi!)