Perfect Quality

This won't be of much interest to most people, but if you own a pool or spa and have problems with maintaining water quality it could be invaluable - some of the maths required is a right royal pain in the arse if you're not used to it. 

The program also allows you to graph levels of the various chemicals over time (this does assume of course that you have the apparatus required to measure them).

Little disclaimer here.  Although the chances of something going wrong when you use this tool correctly are far lower than if you do the maths yourself (given that unless I'm using an original Pentium I only need to get the maths right once in the code and you have to do it each time) do NOT blame me if you jump into your nice new pool and come out with no skin.

(My old practice used it for a hydrotherapy pool for at least a year with no problems).