The rather fine calendar at Rainlendar can be installed as a portable app without modifying the binary at all. See
I'd like to suggest that this fine application be referenced or linked from this site, along with these notes, because it deserves to be more widely used.
I'm sure John will add it when he has time.
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To use Rainlendar portably, install it and run it once. Copy the files from the installation folder to your flash drive or other device.
Next go into your application data folder (just type %appdata% in the run dialog). You can now delete the rainlendar folder in %appdata%, and rainlendar will use the settings in the rainlendar.ini now located in the program folder.
To reduce the size of rainlendar, you can delete any skins you don't want, and also language files you don't need.