Free community-based question-and-answer services are emerging as the latest trend in popular research in the US.

Amazon's Askville service joins a growing list of websites where users can ask fellow surfers to answer a burning question for free. 

Other similar services include Yahoo Answers and the recently launched beta of Microsoft Live QnA. 

Askville researchers earn a virtual currency called Quest Coins, which can be redeemed at a forthcoming Amazon site called Questville. 

Points are awarded for questions and the quality of answers, but are deducted for answers rated inadequate by others users.

However, Quest Coins have already attracted criticism. An Askville Blog posting noted: "Some users have expressed concern about users asking numerous questions just to earn Quest Coins. 

"If you read our FAQ, you will discover that the best way to earn Quest Coins is to provide great answers. In the long run, your hard work will be handsomely rewarded."

Askville users are also encouraged to integrate external resources into their answers, such as YouTube videos, Google Maps and Amazon products.

The key to Askville's success is the free nature of the service. Google was forced to close down its paid-for Google Answers at the end of November owing to low usage levels.

This had allowed users to pose questions for a small fee, and an approved list of researchers, all vetted by Google, could prepare in-depth answers.