Someone asked me if we build widgets on the sprout builder platform, we do.  Sprout builder is a decent "do-it-yourself" widget tooklit that's okay for creating "single use" widgets. We generally point clients to a better solution, such as our Widgetmatic widget management platform. When Sprout was free, building simple easy widgets on it was a viable options for many small companies. With Sprout's new pricing, it is becoming a less attractive widget builder.  Sprout Builder and Sprout Mixer are both decent products but are really aimed for use by small to medium sized business who don't have complex widget needs. Enterprises or divisions of larger firms can probably better benefit from a comprehensive, scalable and flexible widget content management and deployment platform like the Widgetmatic 600/630/650 series.

Sprout builder does have a development SDK but view developers have integrated their services for Sprout (I believe they only Twitter, Yahoo Maps, Google Charts/Forms, PollDaddy, Chipin and Seemic - versus the 30 or so directFrom(tm) servies available for the Widgetmatic platform).