Sentiment is a bulletin board which uses Bitcoin for a Sybil-attack-resistant message ranking. User which supports a particular message can sign its hash using one of his addresses which holds some amount of bitcoins and submit this signature to Sentiment. Sentiment collects all signatures, computers a total amount of bitcoins which support a particular message and uses that for ranking. The idea is that this ranking corresponds to the sentiment of Bitcoin users weighted by the amount of bitcoins they own.

http://sentiment.memeexchange.com/

Note: Signing a message with your private key might have some security and privacy implications, effectively the same as Bitcoin address reuse. Publishing signatures increases risk in case of use of flawed wallet software or discovery of ECDSA vulnerabilities.