Google’s widget link scheme definition was widely accepted. But now a lot of website owners would have wrinkles on their forehead. Google has defined its link scheme, a lot more specific in an update to its Link Schemes page to clarify what types of links in widgets violate Google's webmaster guidelines.
Links embedded in widgets that are distributed across various sites, for example:
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Now, Google defines a widget link scheme as:
Keyword-rich, hidden or low-quality links embedded in widgets that are distributed across various sites...
The only common thing is the example Google used remains unchanged.
The main effort for Google’s update towards its link schemes description was carried out to eliminate potential for confusion and leave a bit less room for personal interpretation. The old definition implies that any link in a widget is a scheme but the new definition differs here. It provides some factors that will cause Google to view a widget link as spam.
Google's Matt Cutts recently warned against widgets as a winning link building strategy, also suggesting when it's proper to use rel=nofollow for them.
Matt Cutts says "I would not rely on widgets ... as your primary source to gather links, and I would recommend putting a nofollow, especially with widgets."