HTC Dual Port Car Charger
Once again out of stock for some reason, but a great accessory worth buying whether HTC branded or not, us a Dual Port USB Car charger. HTC offers one from their own store, but it's out of stock.
Allowing owners to plug two devices in at once, like the HTC 10 and their HTC BoomBass speaker or some wireless headphones. This has two fast 2.4amp outputs for quick charging speeds while on the go. Since it's out of stock and doesn't do Quick Charge 3.0, we'd recommend one from this list instead.
For those who'd rather wait, here's the link to HTC.com to buy it when it becomes available again.