If you're looking to get an HTC One M8 for Windows without a contract, now would be a good time to do so. As part of their latest "Hot Deal" HTC is offering all of their devices for 20% off, including the One M8 for Windows on AT&T, T-Mobile, or Verizon. That brings the price of the device down from $650 to $520 without a contract.
The discount is available through January 2, 2015. Each customer is limited to one device, and must use the coupon HTCHOLIDAY20 at checkout in order for their purchase to qualify for this pricing.
Note: these phones are only contract-free, and not SIM-unlocked.
Thanks to Superboy Micah for the tip!