Is source code available for developers that want to add capabilities?

Yes. A Plug-in SDK is available here:

http://labs.divx.com/ConnectedDownload

Connected Plug-ins

License Agreement

NO COMMERCIAL USE: This License Agreement grants our community members the right to use the Software downloaded from DivX Labs for personal use only in order to evaluate and provide feedback about it to DivX, Inc. Commercial use of the Software or of the work products resulting from its use is not permitted under the Terms of Use of DivX Labs.

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