The STEP File Viewer supports parts and assemblies, dimensions and tolerances, sketch and supplemental geometry, and datum targets. The Analyzer generates a spreadsheet of all entity and attribute information; reports and analyzes any semantic PMI, graphical PMI, and validation properties for conformance to recommended practices; and checks for basic STEP file format errors.
The NIST STEP to X3D Translator (STP2X3D) is an open-source software that translates a STEP (ISO 10303) Part 21 file (.stp or .step) to an X3D (ISO/IEC 19776) file (.x3d) or X3DOM file (.html). The software is based on the Open CASCADE STEP Processor and written in C++. X3DOM files can be displayed in a web browser.
Source code: GitHub