Yeti is now available for download from yeti3d.com. It is still a proof-of-concept more than a parametric modelling environment, but there is enough going on that others may find it useful – particularly for setting the underlying geometry of models.
The latest version allows the generation of custom objects (shown in the video above). The geometry remains really limited – points, straight lines and circles – until I sort out how arrays of data will work.
Yeti is written in C# and I made the source available at bitbucket in case anyone is curious about how it goes together or wants to make it better.