Torsten Manz has written an HP-15C simulation entirely in Tcl/Tk.
It is is GPL'd, and available from:

 [http://www.hpmuseum.org/simulate/15_1_201.zip]

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The link above ist version specific. Generic link is http://www.hpmuseum.org/simulate/simulate.htm - Torsten
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The version available from "The Museum of HP Calculators" is outdated.
The latest version (1.2.09) is available on the simulator home page [http://hp-15c.homepage.t-online.de/].
This version also supports Windows Vista.
Torsten