I'm building a site that needs to guarantee user reputation scores are accurate by preventing users from creating more than one account, at the cost of decreased user signups. So far, the only solutions I have thought of are allowing users to:
- Link to their (verified) PayPal account through PayPal Account Authentication
- Provide their PGP public key, and checking that the MSD (a metric of trustworthiness) is below a certain value
Of course, even these methods aren't bulletproof, but are likely to make creating a sock-puppet account very difficult. Are there any others I haven't considered?