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NewEgg.com has the Jabra SP5050 Bluetooth Speaker Phone for $29.99 with free shipping. when driving ensures you stay on the right side of the law, but still free to make and receive calls.
--12 hours talk time, 200 hours stand by time, night driving mode, auto off
--Autopairing, reject & answer and end calls, weight 2.65 oz

NewEgg.com has the Jabra BT3030 Bluetooth Stereo Earset for $36.99 + $6.99 shipping.
It pairs with your Bluetooth stereo device (typically your phone) and plays back in full stereo. It has an integrated microphone so that you can take calls as well.

Buy.com has the Samsung Black Ultra-Slim WEP350 Bluetooth Headset for $44.99 - $15 rebate (Exp 06/30) = $29.99 with free shipping.
Features include up to 5 hours of talk time, up to 100 hours of standby time, and weighs only 7.1 grams.

eCost.com has the AZiO USB 2.0 Micro Bluetooth Adapter for $20 - $20 rebate (Exp 05/31) = Free + shipping.
It's pretty small, but not as small as some of the other mini adapters out there.

Buy.com has the Soyo FreeStyler 500 Bluetooth Headset BTWLHS01 for $10 with free shipping. Supports Bluetooth 1.2, Operation range Up to 10 meters (30 feet). Talk time up to 6 hours, standby up to 180 hours.

Buy.com has the Jabra BT135 Bluetooth Headset -recertified for $13.99 with Free Shipping. New Users apply the google checkout and get the $10 Off for $3.99 with Free Shipping
The lightest and most compact Bluetooth mobile headsets in its category.