Griffin HandsFree Mic and AUX Cable - $10

Just sold my car, so I don't need this anymore. Moving away in a week.
It costs 29.99 new. Works perfectly

Features

Cable Length: 35.4" (90 cm)
Type: Stereo audio cable, 1/8" stereo minijacks
Microphone: high-sensitivity condenser
Compatibility: iPhone 4, iPhone 3GS, and iPhone 3G
Requires your iPhone to be running iPhone OS 3.0 or higher
Technical Specifications

Handsfree microphone for iPhone & many other smartphones
Doubles as an AUX cable for high-quality audio
Zero-setup, zero-configuration; just plug, play and talk
Incoming call pauses the music on your iPhone and sends call to your car's speakers
Handsfree mic and inline switch make it easy to answer and talk while you keep both hands on the wheel
Slender plug design so you can keep your iPhone in its case when you plug in