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