I want to buy headphones/earphones/IEMS/listening device on Amazon.
My budget is $20. What do you guys recommend?
 
http://www.amazon.com/gp/bestsellers/electronics/172541/ref=pd_mw_e_mte
 
 

Sennheiser HD 202 Dynamic Supra-Aural Headphones?

Sennheiser HD201 Lightweight Over-Ear Binaural Headphones?

Sony MDR-XD200 Stereo Headphones?

Sony MDR-V150 Monitor Series Headphones with Reversible Earcups?

Sennheiser CX300-W Earbuds (White)?
 
 

They seem to be different types of headphones (ie over ear, around ear, in ear.)  What type are you looking for?  That should narrow it down.  Might also be helpful to see the amazon reviews, as the amazon reviewers of headphones in that class are not totally off-base.  From a quick perusal of the list, the porta pros are a cut above the others, and are generally respected as a decent entry level headphone, although at 29.99, they're 10$ more than you want to spend.  

If you want IEMs and can go another $9.50 and not Amazon I would suggest Brainwavz M1 $29.50 free shipping
 
http://mp4nation.net/catalog/index.php?main_page=product_info&cPath=51&products_id=399