I want one because I've read that you can remove hissing with them. Turn the volume down on the cable until there's no more hissing, then turn the volume up on the source (e.g. computer).
Radio Shack is selling them for about $10 which seems like a complete ripoff.
This one is the cheapest: http://www.ebay.com/itm/New-3-5mm-M-F-1M-Stereo-Headphone-Audio-Extension-Cord-Cable-with-Volume-Control-/270831486718?pt=Other_MP3_Player_Accessories&hash=item3f0ed086fe#ht_3582wt_1270
This one looks nicer, but I can't see how the volume control works: http://www.ebay.com/itm/3-5mm-Stereo-Headphone-Audio-Extension-Cord-Cable-with-Volume-Control-For-iphone-/150831987892?pt=Other_MP3_Player_Accessories&hash=item231e497cb4#ht_2829wt_1037
What do you think?