REDWOOD CITY, Calif. (AP) _ The cacophony of the Internet reaches new levels Monday.

Third Voice is introducing a new software program that can be downloaded for free that will enable Web surfers to add their comments to any Web page like editors armed with highlighter pens and sticky notes.

``Third Voice creates for the first time a system of checks and balances on the Internet,'' said Chris Shipley, editor of DemoLetter, a technology newsletter. ``There is now a way to communicate at the point of context, which has tremendous implications.''

Not everyone is going to enjoy the effect.

Web shoppers can stick a note on a retailer's site to complain they've seen the same item for $10 less elsewhere. Financial pundits can add warnings to corporate earnings reports. Voters can contradict politicians, alternative healers can rebut doctors.

``Don't buy this,'' said a note that popped up during a test of the system. ``This product is terrible.''

Here's how it works:

First, you download Third Voice software from its Internet site and install it on your computer. When you want to add a comment to a Web site, you simply click on an icon on your screen, type in your message and post it on the site. Third Voice messages can be inserted on any page where there is text. The creator of the page can't remove your comment.

The software also allows you to see the comments of others. If too many comments are attached to one spot, an icon will appear on that spot. Clicking the icon will give the reader a list of all comments, 10 at a time.

If you don't want to see any comments, you can simply turn Third Voice off.

Third Voice CEO and co-founder Eng-Siong Tan says the company won't restrict most comments, but an editorial team will take complaints about offensive or illegal speech from users of the service and assess whether the comments should be removed, similar to the way America Online polices its sites. In addition, comments will only be allowed from individuals, not corporations.

``We're not going to let Chrysler go on Ford's Web site and post a note,'' said Tan, acknowledging enforcement may be difficult considering the potential for a vast and growing number of comments.

The company will also offer a Web directory service that will rank Web sites by the number of Third Voice comments. The directory will carry advertising, which Tan hopes will generate income to support the venture.

But making money is his secondary goal, said Tan. ``We think of this as a small step back to the original meaning of the Web: Free, open expression for all.''

Barry Parr, research director at International Data Corp., said Third Voice ``could be a breakthrough product.''

But Parr said the company faces significant challenges. The first is a chicken-and-egg issue. Initially Web surfers may not bother to download the Third Voice Software because the number of people posting comments is so few.

``They also have some potential problems with spam and information quality that are going to be harder to address, and could require a lot of human intervention to resolve.'' That won't be easy for a company with 26 employees.

The technology has received some significant backing, more than $5.5 million in funds from the venture capitalists Mayfield Fund and Draper Fisher Jurvetson.

Third Voice is also offering private forums to groups who want to comment on specific Web pages, but do it without the world looking in.

Using this, teachers could host class discussions, researchers could peer review documents and office workers could share project-related information.

Leo Jolicoeur, who left his job as a vice president at the Web directory Infoseek to run business development for Third Voice, said that businesses could use the software to seek out and communicate directly with their customers.

``In addition to giving every user a voice on the Web, Third Voice also presents incredible appeal to advertisers, sponsors and transaction partners,'' he said.