Which programs do you actually use in your calculator?

[DA74254]

Depending on which calc you mean and "how often" it is used..

HP Prime:
AngStar (Angles between stars in astronomical use)

HP 35s:
Mostly equations

HP 28S/50G:
MPG/L10K (Conversion between miles per gallon / litres per 10 km)

42 (any flavor):
RelHumDew (Relative humidity and dew point based on wet and dry thermometer)

Apart from Prime prog., all self made and freely available upon request.