Many CAD programs support both the STEP and STL format and should support step to stl conversion just fine. This is usually done for the purpose of printing the model on 3D printer, so called stereo-lithography. The very same can be probably also done with some dedicated converters.
Open STEP file Open STL file3D viewer and converter with huge file type support
A website STL converter
A Mac version of the popular 3D visualisation software
A set of 3D modeling tools
A converter for 3D formats
An online CAD conversion service
Creo Parametric (formerly known as Pro/ENGINEER) is a complete 3D CAD software solution by PTC
A CAD visualizaton, analyzation and translating system
An open source multiplatform 3D CAD/MCAD/CAx/CAE/PLM modeler
A commercial online CAD converter
A 3D CAD translation software