net10byop.com and straighttalkbyop.com. My tipster says LTE phones can be activated but isn't sure it LTE will work.
It looks like it's true. Both sites used to say that:
"This program is not compatible with CDMA carriers Sprint or Metro PCS, 4G LTE devices, iPhones, BlackBerry, data only devices, lost or stolen devices, branded TracFone, SafeLink, NET10, Straight Talk or Telcel America handsets"
Now they just say:
"This program will not work with Blackberry or iPhone, nor with Straight Talk, TracFone, Safe Link, NET 10 Wireless or Telcel America brand phones."
I tried it myself with the ESN of a broken Sprint HTC EVO and got as far as the site asking for my credit card. I didn't go any further because I don't have a working Sprint phone to try.
If you try activating a Sprint phone on NET10 or Straight Talk let us know in a comment if it works or not.