The most popular open format dealing with BIM is IFC format, that it is maintained by buildingSMART association.
To date, there have been several releases for the format, most of them now obsoletes.
We can consider the IFC2x3 as the last ‘stable’ version, and the IFC4 as the ‘nightly’ builds. In the near future there is the IFC5 to come, but is still in early planning phase.
For the IFC4, the last one is called IFC4 Addendum 2 (plain: IFC4 Add2 and more informally IFC2x4), released in July 2016. Incorpores some improvements that had been requested before starting the IFC4 certification process, so it can be considered as buildingSMART Final Standard baseline for IFC Reference View V1.1 and IFC4 Design Transfer View V1.1. The express schema can be reached here, and the HTML documentation can be downloaded here.
First deal is to have converted all the EXP schema to VBA Functions/Enumerations/Entities, so it can be called from inside IDE. They’re nearly 90.000 lines of definitions, so it’s a huge work.
Second deal is to have Entities/Functions converted to somehow Graphical interface (Dinamo/GrassHoper alike), so they can be generated, linked, referred and programmed to do “things”.