Free42 HP-42S Calculator Simulator is an Open Source recreation of the HP-42S Scientific Programmable Calculator and HP-82240 Printer.
Free42 has two calculators, binary and decimal. It also has two skins, realistic and standard. Other actions include the option to import or export programs, clear or show the printout. Preferences include settings like single instance, output to gif or text, keep the window on top, and more.
Free42 looks identical to the HP-42S, at least as seen here. Aside from that, you push buttons. Our math skills are right in line with our graphics and web design skills, so the only other thing we can say is it also seems to work well.