Photo-related web widgets dominate the list of Top 10 widgets worldwide, led by Slide - its widgets accounting for 117 million unique viewers in April - according to comScore, which also announced the launch of its Widget Metrix service, reports MarketingCharts.

Slide's widgets reached 13.8 percent of the total worldwide internet audience in April, when all widgets reached some 178 million users, or 21 percent of the internet audience worldwide, comScore said.

Other top photo-related widgets were RockYou (82 million viewers), PictureTrail (31 million viewers), and Photobucket (28 million viewers).

Facebook.com's recent announcement that it is opening doors to third-party widget developers will likely have a significant impact in shaping the widget landscape over the coming months, comScore said.

"Third-party audience measurement is critical to the emerging widget economy," said Hooman Radfar, founder of widget delivery platform Clearspring. "Measuring and understanding the widget universe is an important first step in enabling publishers and advertisers to fully realize the benefits of the burgeoning distributed Web."

MarketingCharts provides more data from the study.