CATIA is certainly the best software for catpart conversions or exports of any kind, but CATIA's format are well-supported in 3rd party 3D modeling software or even some dedicated converters. This type of catpart to stl conversion is typically done for the purposes of 3D printing.
Open CATPART file Open STL fileA set of 3D modeling tools
An online CAD conversion service
A CAD converter with huge format support
A CAD visualizaton, analyzation and translating system
A commercial online CAD converter
A 3D CAD translation software
A dedicated CAD converter