NEW YORK — Picky eaters used to have few choices about expressing their delight or disappointment over a restaurant meal: Say something to the waiter, tell their friends or fill out that little comment card that comes with the check and hope somebody reads it.

These days, many of them are going home and firing up their computers.

Pioneered by food and restaurant discussion boards like Chowhound.com, a handful of Web sites have taken amateur restaurant critiquing to new levels, giving diners more power than they ever had to sound off on what they like and don't like. And chefs are taking notice.

There's a lot at stake for restaurants, whose reputations can rise or fall on a strongly worded review in an influential local media outlet. With often tight profit margins and a high rate of failure, restaurants have keenly watched reviews that appear in local media — and now, from average users online.

www.Yelp.com, a San Francisco-based startup company, has made a name for itself as a user-driven forum for sounding off on local businesses, especially restaurants. Yelp displays detailed information about each commentator, including their other picks and pans, giving readers a chance to decide how much credibility to give any particular review.

Jeremy Stoppelman, who founded Yelp in 2004, said he came up with the idea after concluding that word of mouth was the best way to find restaurants he liked. While they may not be professional reviewers, Stoppelman says the sheer volume of information from its users — he calls them "Yelpers" — makes up for the fact that there's not a single, professional editorial voice.

"Yelp just democratizes the reputation of a business," Stoppelman said by phone. "Rather than a single arbiter of taste, it's hundreds of people saying whether they like this business or not."

Yelp spokeswoman Stephanie Ichinose declined to say how many registered users the site has, but she said Yelp gathered more than 500,000 reviews since January, and well more than a million since launching in late 2004. In May it had 4 million unique visitors.

Like other restaurant guides such as Zagat, Yelp has measures in place to ward off shilling by affiliated parties. But also like Zagat, Yelp won't say what those measures are, for fear that their disclosure would encourage fraud.

Zagat Survey LLC, a restaurant guide company started 28 years ago, is also based on user reviews, but in book form, with only short excerpts pulled from each review. Zagat's Web site started off as subscription-only but has increasingly made more information available for free, except for the numerical ratings of specific restaurants.

Dan Entin, senior product manager for Zagat Survey's interactive products, notes that there has been a surge in competition for online restaurant reviews in the past few years. Responding to what he called an insatiable appetite for more information, Entin says Zagat also began displaying reviews online from individual members in August of last year.

www.MenuPages.com and CitySearch, a unit of the publicly traded company IAC/InterActiveCorp, also allow users to post comments about restaurants, but Yelp's look and feel veers closer to that of an online social hangout like MySpace.

Reviewer "richard p." from San Jose, Calif., who has a green-tinted likeness of North Korean leader Kim Jong Il wearing sunglasses on his profile, says the wait at Mario Batali's restaurant Lupa in Manhattan was "killer" but "well worth it."

Chefs, meanwhile, are taking note of what goes on. "It's input and information. We have to look at it," says Eric Tanaka, who oversees five restaurants in Seattle for local chef Tom Douglas. "It's a tool for us. Sometimes it's flattering, sometimes it's not."

Tanaka said some of the reviews may veer toward the emotional, but he tries to look past that to see if there is a pattern emerging about a potential problem that needs to be addressed.

"We look at it for trends," he said. "If there are three people who say they don't like the steak, we look at it. If it's one person going off on a tangent, we put it in perspective."