Since I get asked this question a lot, here is a quick round up of the more popular widget marketplaces, builders and distribution platforms (at least the ones that matter most):

Name	SDK	Builder	Sharing	Tracking	Gallery	Ad Option
ClearSpring	Flash, Javascript	No	Yes	Yes	No	Yes
Gigya	Flash, HTML	No	Yes	yes	No	Yes
iWidget	No	Yes	No (1)	No	Yes	No
SpringWidgets	Flash	No	Yes	No	Yes	No
Sprout	Yes (2)	Yes	Yes (Gigya)	Yes (Gigya)	Yes	No
WidgetBox	Config Only	No	Yes	Yes	Yes	No

1. Widgets to not appear to feature a full range of sharing options
2. SDK is not publicly available and adds functionality to the SproutBuilder