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AffordaBilly is a simple interest loan calculator, similar to "VeryInteresting" but focused on determining how large a loan the user can afford, given an interest rate and length of the loan in years. Users can choose monthly or bi-weekly payments. The results will show the maximum amount of a loan that the user can afford based on the monthly payment and interest rate. This is a very simple, yet nice educational tool for showing just how much only 1 or 2% interest can make a difference over the course of time.
Price: Freeware

ColourCalc is compact yet powerful tool that quickly converts between HEX and Decimal RGB colour modes, and includes a 216 colour web-safe palette for web designers. The Pro version also includes CMYK support, and has colour adjuster sliders for fine tuning.
Price: £25

This program calculates the distance between two zip codes in the United States. The sub-routines included in the code can be used to calculate any distance between latitude and longitude coordinates. The package includes data for the US, the script includes two sub-routines, one gets the coordinates and city name, the other calculates the distance between two points. The result should be very precise because the calculation takes into account the fact that the earth is not a perfect sphere.
Price: $74.00

PayBackTime is a simple perl/CGI interest loan calculator, similar to "VeryInteresting" and "AffordaBilly" but focused on determining how long it will take to pay off a loan or credit card, given a minimum payment, balance, and interest rate. The results will show the time it will take to pay off the balance with that monthly payment, number of payments, and the total interest the user will end up paying.
Price: Freeware

Quadratic-O-Matic is about the simplest program imaginable to set up and to use. You simply call the program as a url such as http://www.linguistic-funland.com/scripts/Quad/quadratic.cgi, type in the numbers to get your quadratic equation, and click "Solve that puppy." You will then be given the roots of the equation.
Price: Freeware

The Air Duct Calculator is a PERL/CGI script which allows users to size ductwork and calculate duct pressure losses. The program is written in the PERL programming language and is intended for use on the World Wide Web via the Common Gateway Interface. The program is designed for use by Mechanical Engineers who design air distribution or exhaust systems for Heating Ventilating and Air Conditioning systems.
Price: Freeware

This program allows users to input a loan amount, an interest rate, and the number of years of the loan. The program then outputs the monthly mortgage or loan payment. The program uses the single payment compound amount equation. Version 2.0 will include the single payment present worth equation, the sinking fund factor, the capital recovery factor, the uniform series compound amount factor, and the uniform series present worth factor. I have a working demo of version 2.0 which you can check out. I also made a script to amortize loans.
Price: Freeware

This calculator is used to calculate pipe friction losses for straight piping handling water and water with varying percentages of ethylene glycol. The calculator uses an algorithm developed from the basic principles of fluid mechanics. The program is based on the Darcy-Weisbach equation, the Colebrook equation, and the Moody equation. The program is intended for use only on straight piping. The program does not include pressure losses due to elevation differences, or flow through fittings.
Price: Freeware

This calculator is used to compute the thermodynamic properties of moist air. These properties are used by mechanical engineers to design Heating Ventilating and Air Conditioning (HVAC) systems and processes.
Price: Freeware

This calculator is used to compute the thermodynamic properties of saturated steam. This calculator uses steam properties from the 1967 ASME Steam Tables, published by the American Society of Mechanical Engineers. This calculator allows the user to input either temperature or pressure in either metric or I-P units. The calculator then outputs the pressure or temperature, the specific volume, the enthalpy, and the entropy. The values are obtained by a linear interpolation between available data points. This calculator provides reasonable accuracy.
Price: Freeware