nonpareil is a micro-assembler and simulator package for the vintage HP calculators. It's written originally for Linux by Eric Smith.
Here you can find nonpareil partially ported to Mac OS X 10.4. It runs fine under Mac OS X Leopard 10.5.x as well.
nonpareil is made available under the terms of the Free Software Foundation's General Public License, Version 2. More information about Eric's nonpareil you can find here.
If you like nonpareil for Mac OS X you can donate it via PayPal (using PayPal account or by credit card)
Otherwise you can donate Nonpareil for Linux at Eric's site.