While it's fun to see non-skeleton, Marvel Legends-esk style articulated 3D figures being made available, I feel it's even better to see open source versions made available on Thingverse.
Designer Jason Welsh released both a male and female articulated figure models for download, which might give some assistance to others designing future versions.
2026 Update: All links updated to the Internet Archive where possible. In the years since this post, T13 (or "Dummy 13" printable action figures have emerged as well:
https://www.printables.com/model/981111-dummy-13-version-10