HTC One M8 users outside of the U.S. and Canada get to enjoy the Extreme Power Saving Mode. Unfortunately this feature was not enabled for HTC One (M8) units in North America. When this mode is enabled, non-primary functions of the device are switched off, thus saving your battery life. According to HTC, this mode can save an an extra 24 hours of standby time, taking into consideration the odd call or text.
The FCC was concerned about the ability of the phone to make emergency calls in the low power mode. But the FCC is now satisfied, and the Extreme Power Saving Mode is making its way to U.S. and Canadian versions of HTC's new flagship phone, as a 149.85MB update. To get to the update, you need to go to Settings > About > Software and see if you can grab the update manually.
Besides adding the Extreme Power Saving Mode, the update also comes with some goodies for the camera and the gallery. The new build number is 1.55.631.4, and HTC suggests using a Wi-Fi connection to download and install the update.