Quoted from robin:The easiest authentication system I can think of, which is easy for me to implement and also easy to use/understand for most Pinsiders would probably be a simple SMS containing a short verification code, sent to a registered phone number.
Downsides:
- costs, although it looks like Twillio would only set me back $7.50 / 1000 sms codes
- folks would need to add their phone # to their pinside account
Some people (yours truly) still don't have cell phones, so SMS messages can be a pain. I do have a google voice number for this specific purpose, but some sites won't send text messages to those.