Planet Waves 1/8" Stereo Male to 1/4" Female Adapter

 stereo mono issues
Mono and stereo are determined by what you're plugging into it. It is stereo if you connect a 1/4" stereo male end - mono if you connect a 1/4" mono male end.

 The female end is mono
Just a heads up, since it's not clear from this page or the seller's web site: the female end of this adapter is mono. So this will work for electic guitars and such, but will not be want you want for headphones. (It's 3 stars since it seems to be a fine product, but was not what I was looking for. I haven't tested it.)

Also, this product is not returnable to Musician's Friend.