Free Web 2.0 Ideas First in a series.→[More:]
Ingredients:

1) A keychain sized device that can read barcodes or RFIDs, can receive wireless signals, and can use wireless/bluetooth/infrared to communicate with a mobile or PDA
2) Printable tags or RFIDs easily applicable to goods and places where services are sold, purchasable by individuals or merchants and non-conflicting with their own internal systems
3) A set of websites where people are commenting about products and services and rating them
4) A group of merchants who are physically their products/services with tags/RFIDs readable to the device
5) A piece of software that translates web comments/ratings of products and services to something interpretable by the keychain-sized device, perhaps using RSS feeds or something similar

Recipe:

1) Sell the keychains and tags to people who want to mark up their cities with them in their hipsterish 2.0 way
2) Promote the tagging to small merchants, banks, nightclubs, subway stops, etc. who can actually benefit by offering more transparency to their customers
3) Promote subscription to the keychain service to websites
4) Develop an SMS based system where people can upload comments whilst in the wild, away from their computers, to the review websites
5) Profit!

(it's not a good idea, it's a FREE idea.) :)