[jyl]

For a geeky afternoon
You can run DOS on the iPhone/iPad via iDOS - though you may have to jailbreak, situation keeps changing
Emulator for Voyager series HP11/12/15/16
Your 11C, 12C, 15C and 16C into a sleek single iPhone App! Available on the AppStore at $14.99/€11.99/£8.99
Emulator for HP35/80/45/55
ClassicHPCalc for iPhone, iPod touch, and iPad on the iTunes App Store
Emulator for HP41CX
i41CX+ RPN Calculator with Printer for iPhone, iPod touch, and iPad on the iTunes App Store
Free42 for Android
Free42 for Android - Project Status
uCalc project
EEVblog #130 – The uCalc Credit Card Scientific Calculator / Computer @ EEVblog – The Electronics Engineering Video Blog
uWatch project - all sold out, v2 ETA unknown
uWatch - A DIY Programmable Scientific Calculator Watch