Republican Calendar is a widget that shows the Republican Calendar date (following the 4/128 rule for the leap years) and the decimal time. It is free software offered under the GPL.
The images appearing on the widget were taken from the Republican Calendar of 1794 drawn by Louis-Philibert Debucourt, and they are on the public domain.
Using the 4/128 rule might not be the best choice from a historic point of view. But as it was explained before, since there are no clear rules, and this one is the most scientifically accurate and universal for our times, not depending on the equinox in Paris, nor in Gregorian rules, I think the 4/128 rule is a good choice.