For now, the Widget Metrix service is only tracking widgets--mainly photo- and video-streaming players--that can be embedded on Web pages like blogs and social networking pages, rather than desktop widgets. (Notably, YouTube's video players are not being tracked by comScore.)

Still, over 177 million consumers were exposed to widgets worldwide in April, or 21% of all worldwide consumers online--a trend being driven by the current flood of user generated content, according to Linda Boland Abraham, executive vice president at comScore.

"The recent explosion of user-generated content has helped create a worldwide marketplace for widgets," said Abraham.

At the moment, consumers prefer widgets for sharing photos with friends and family, comScore's Widget Metrix finds.

In April, photo-sharing widget maker Slide was the top widget provider with a worldwide reach of more than 117 million unique viewers, or 13.8% of the total worldwide Internet audience.

Other top photo-related widgets included RockYou with 82 million viewers, and PictureTrail with 31 million viewers. Also in the mix with 28 million viewers was Photobucket, which was recently acquired by Fox Interactive Media.

Of note, the popular social networking site Facebook recently opened its doors to third-party widget developers, which is expected to further boost their value.

While business models are still an afterthought for some widget makers, many are looking toward bright futures as ad networks--a goal the new third-party user-measurement will help facilitate.

"Third-party audience measurement is critical to the emerging widget economy," said Hooman Radfar, founder of Clearspring, a widget maker that just struck a deal with NBC Universal to carry the network's content throughout the Web.

Added Kevin Freedman, chief financial officer at Slide: "Measurement is really going to shine a light on our young and attractive audience for advertisers."

* Penetration = Number of unique widget viewers /number of total Internet users for that region