It seems there is several options how to perform COB to STL conversion. Both are 3D/CAD file types so you should have no trouble with converting Caligari 3D object files (*.cob) as Stereolithography files (*.stl). For example Accutrans 3D or CopperCube can probably make this conversion happen.
Convert Caligari trueSpace 3D object file to Stereolithography CAD file.
We found 3 software records in the database related to .cob to .stl file format conversion.
Microsoft Windows software - convert cob to stl on Windows
A CAD 3D geometry translation software
Convert from cob file format
The tables with software actions are a good pointer for what a certain program does with various file types and often may give users a good hint how to perform certain file conversion, for example the above-mentioned cob to stl. However, it is not perfect and sometimes can show results which are not really usable because of how certain programs work with files and the possible conversion is thus actually not possible at all.
A 3D CAD translation softwate
On-line services software - convert cob to stl on web
A free online 3D model conversion service