I have an HP-41CX+ app. I think I was using a free or entry version and I mentioned it in a review so the author sent me an upgrade to the full CX+ version. It's pretty great although it's complete overkill for me. It obviously lacks the feel of the physical calc but mine died years ago.

There's a good HP-42 app as well although I'm just more used to the 41 so that's what I use.