Giles Harrison Software
This does the computation on wet bulb and dry bulb temperatures, to derive Relative Humidity, Dew point and vapour pressure, for aspirated and screen psychrometers. [Windows]
Saturation Vapour Pressure calculator
This computes the saturation vapour pressure above water and ice, for a range of temperatures between -50 and +50 degreesCelsius using the polynomial expansions in Pruppacher and Klett (1997) Microphysics of Clouds and Precipitation. [Windows]
If you're using the Observatory data system, you'll need to convert between the day number of the year and the date. This little program will do it for you. [Windows]