Pathfinder3D is a free, open-source web-tool for architects of stacked 3D ICs that use through-silicon vias (TSVs)[1]. Pathfinder3D allows early, system-level analysis of floorplanning issues. Users without access to a commercial 3D IC process may evaluate their architectures using the predictive FreePDK3D45 technology (3D-IC Version of the FreePDK for the 45nm technology node). This tool was developed with support from SRC, NSF, and DARPA. Pathfinder3D is available for public use since Feb 1, 2012.