The Mophie Juice Pack for the HTC One looks great, but it's not cheap at $99. The Power Glider from iBlason might be better value and with an additional feature too!
Priced at $59.95 from Amazon US (international availability TBC), the Power Glider is available in black or white and like the Mophie, packs a 2,500mAh battery which can apparently provide 80% of a full charge to your HTC One. A LED indicator on the pack itself indicates the internal battery level.
Like it's competitor the pack is pretty chunky and will largely ruin the lines of your gorgeous device, but it at least has an extra little trick up it's sleeve - it has a kickstand built into the back which might be useful for watching videos etc.
We'll update you with a review as the device becomes available (and we've inquired about international availability) - it'll be intersting to see if the quality is up to scratch.