They have versions for iPhone, Windows, PocketPC, Linux, Mac, etc..

http://free42.sourceforge.net/

It doesn’t hurt that it copies the actual calculator I’ve been using since 1988.

If you need the financial functions or are attached to the particular interface these may be worthwhile. Two important questions I can’t figure out from the descriptions are:
1. Are there click sounds for key presses that don’t require earphones on older iPod touch versions?
2. If you write a program, can you store it?

I’ve been using http://appshopper.com/utilities/digits, which does have click sounds and has a lot of functions available with a pull-strip (what looks like a corduroy barcode in the photos).

Since there are many Tablet PC users here, I should add that on my tablet I use http://www.tordivel.no/xcalc/, which is free. On the tablet it is much more clear when you have actually hit a key, a problem on the iPhone unless you have click sounds that are loud enough.

HP finally gave finally his blessing to sell it.
Giving back the author a bit of credit ;)