This isn't new, but Microsoft released it's Threat Modeling Tool v3.1 as a public beta a couple of weeks ago. I've been using this tool internally at Microsoft for a bit longer than that, and it is impressive.
Instead of being a comprehensive tool for experts only, this tool makes threat modeling approachable to the average developer with little security background. At its core is a hosted Visio data flow diagramming tool. The tool produces workable diagrams which are then automatically analyzed to suggest common threats.
Adam Schostack presented a demo of the software at the Bluehat Conference in October as part of a point/counterpoint session (Adam begins about halfway through). Don't blame me for being too far behind, though -- the video was just posted to technet before Thanksgiving.