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Jawbone Icon HD
The Jawbone Icon HD is the best, clearest, most comfortable, and easiest to use Bluetooth headset I've used.
Call clarity is taken care of by the military-grade NoiseAssassin 2.5 noise-cancellation and wind reduction technology. The built-in 10mm wideband speaker allows you to privately listen to music, podcasts, YouTube videos, sports, phone calls and more, all in full-spectrum, HD-quality sound.
To top all that off, the Jawbone HD supports a whole range of downloadable apps and voices.
Image credit: Jawbone
Belkin TuneCast Auto Live
If you're going to carry around your entire music collection with you wherever you go, it makes sense to be able to access that resource when you're on the move. One place where it's great to have access to your tunes is when you're in the car. If your car stereo isn't equipped with Bluetooth, then the easiest way to pipe your tunes from your iPhone to your car stereo is by using an FM transmitter.
By far the best FM transmitter for the iPhone that I've used is the Belkin TuneCast Auto Live. Featuring ClearScan technology, it can scan for the clearest FM frequency for you to use. The optional iPhone app can be used to recommend the best frequencies for you to use as you travel. TuneCast Auto Live is also the only FM transmitter with GPS-assisted frequency scanning, allowing for the best audio quality when you drive.
As an added bonus, this device will keep your iPhone's battery charged up while you use it, so there's no need for a separate car cord.
Image credit: Belkin
iPhone SLR Mount
Without a doubt the craziest photography accessory for your iPhone is the iPhone SLR Mount which allows you to couple a Nikon or Canon SLR lens to your iPhone 4/4S.
This case-adapter combo lets you mount a Canon EOS or Nikon SLR lens to your iPhone, giving you access to powerful photographic techniques such as depth of field and manual focus otherwise not available on the iPhone.
The whole arrangement is quite unwieldy, but it is capable of taking some fantastic photographs. Eat your heart out Instagram users!
The only drawback of this device is price, but if you are the sort of person who has Nikon or Canon glass laying about the place, then you're probably not going to be that shocked by the price tag.
Note that this is only the adaptor and that you don't get a lens thrown in for that price.
Image credit: Photojojo