There's a short article up on BMighty.com, which bills itself as a site for "practical technology expertise for growing companies," that provides a couple good examples of companies (somewhat outside the traditional online media space) using widgets to successfully market their products. 

There's a quote from Liza Hausman, VP of Marketing over at Gigya, who says that the key to a successful widget is to spread the word about it and encourage downloads; in her words, "Create something that's relevant and makes consumers want to interact with you."  Well said.

You can read the article in its entirety here.