In the past week, thousands of people have succumbed to the temptation to squirt their friends -- virtually -- with a message from a ketchup bottle.

Pittsburgh's condiment maker H.J. Heinz Co., traditionally in the business of selling the real stuff, is venturing into the trendy social networking arena. A Web site revamp lets fans "splat" friends with e-mail messages written in ketchup, sent either directly from the site or through a tool installed on their Facebook or MySpace pages.

This is Heinz's first use of a widget, but the little tools have been driving one of the faster growing trends in the marketing business. Trade publication eMarketer recently estimated U.S. companies would spend $40 million on widgets this year, up from last year's $15 million.

Basically, a widget is a software program that appears as an image on the user's page and can display information or make it easy to access an application.

Among the more than 4,000 widgets on the Yahoo! Widget Gallery are tools to push a daily Calvin & Hobbes comic strip to users, previews for the TV show "Lost," and even National Hockey League schedules. There also are widgets tied to IBM, Disney, Builder Magazine and cosmetics retailer Sephora.

These kinds of tools are commonly used on popular social networking sites such as MySpace and Facebook. Companies see the traffic to those sites and want to be a part of it, said Michael Walsh, assistant professor of marketing at West Virginia University.

Joining a casual conversation between friends is good for a brand, so if a company can find a fun or useful tool that people want to share, marketing can build a life of its own.

CareerBuilder.com launched Monk-e-Mail early in 2006, allowing users to send goofy monkey-delivered images and messages. Though a different program was launched a year later, the monkeys have kept the company's name circulating. A company spokeswoman said 130 million e-mails had been sent so far and that 20 percent of those who received one turned around and sent one to someone else.

Uptown company sunKing Digital, which created the Heinz tool and revamped the Web site, stands ready to come up with new ideas if the first widget begins to lose momentum. The designers had lots of ideas, said Jeff Myers, executive creative director.

For now the viral tool seems to be doing its job. "We're thrilled with the response," said Heinz spokesman Michael Mullen, who said thousands of people had used it in the first five days.

The company has listed a group of predetermined messages that consumers can send via virtual ketchup but eventually that could change. Getting the animation right is important, Mr. Myers said, as is understanding how consumers actually use the widget.