Widget spam is a concept that came up a couple of years ago when site owners began distributing widgets for their fans to place on their own pages. The widgets contained links. Some of those links were considered spammy by the search engines. The result? Lots of penalties for the sites that were caught out.
Widgets became very popular as a way of distributing links and spreading information about a site. It’s easy to understand why. A widget is basically any piece of code that can run a small application independently upon a page. All of the useful little tools that you see around the net, all of the clocks and gadgets, all of the badges, are all widgets. They’re very attractive, and they’re very easy to create.
The area which caused trouble was the embedded links contained within widgets. Site owners would embed links in their widgets for their off-page search engine optimisation. The more successful widgets became attractive to other site owners who would then pay to have their link embedded instead. It got messy. Google became aware of this and started to panic about spam and paid links.
Site owners continue to embed links in widgets, and this continues to be accepted as a white-hat practice of SEO, as long as the recipient is aware of the link’s existence. The line appears to be crossed when the widget’s purpose has nothing to do with your site. Google defines off-topic widgets as spam. If you’re using widgets as part of your off-page SEO, watch your topics for safety and make sure that your widgets are relevant.