[wrycoder]

There is a great HP-41C for iOS: i41Cx. I believe it's the most expensive app I have, and it's worth every penny. I still have my HP-41, but I never drag it out because of this.

[joezydeco]

There's an open-source clone of the HP-42s that has found it's way into different smartphone apps.

I use Free42 for iOS and it works beautifully. Plus I can leave my real 42s safe in a drawer at home.

https://itunes.apple.com/us/app/free42/id337692629

There are other builds for Android, Windows, Mac, and Linux:

http://thomasokken.com/free42/