Anything with line in will work. That makes for a huge selection. Any other criteria that might help to narrow down the options? Budget? Sound quality? Brand preferences? Other features needed/wanted? Does portability or size matter? Do you want bluetooth rather than wired?
You can try JWIN Silver Ultra Portable speakers. Sound quailty is good and clear with 360 degree sound, covering all surrounding areas. It comes with a built-in volume button which makes controlling your cellphone easy and the best part of this is that it is Compatible with any cellphone so for sure you can use it with your droid x