For user generated content sites, widgets are great for letting your existing user base export their content contributions to their other online presences. 

Examples of these types of widgets are numerous – Flickr widgets let you export your photos, Delicious widgets let you display your bookmarks, and Upcoming.org widgets let you display your events.  RateItAll is about to release a widget that lets its members widgetize their RateItAll profiles and review history.

The upside of this sort of widget is that it gives your existing super users the tools to spread the good word about your site.  The downside is that this sort of widget is unlikely to generate much excitement among folks who aren’t already super users of your service.