Long-lived (design) software
Gimpel Software  has an advertisement that they have the longest continuously sold software product in history. It is their "Lint" C code checking program and in fact this product has been around for as long as I can remember - even back to Byte Magazine and IBM DOS days. They still sell it today, although it has been updated to work on Windows and now it not only checks C code, but C++ as well.
Way back in the last century my first PC was an Apple ][. I immediately built expansion boards for it and set about controlling and testing devices with it.
The language of choice then was a C compiler called Manx C. I used C on the Apple and my first Compaq Portable PCs until Microsoft released Visual Basic. I still use C today on embedded designs but have transitioned to C# for PC programming - sort of the same, but not the same products, and I can use what I have learned over the years, but I can't really use the same source.
Likewise the first Mechanical CAD program that I had was AutoCad 2.5 ; this was their first really popular release and I still use a version of AotoCad today, but not AutoCad's as it has too many features that I don't need. I use a clone version called IntelliCAD.
IntelliCAD still uses the simple keyboard interface and commands that I originally learned way back in the 80s. Again the same, but not the same productsl; although I do have some drawings from the 90s and I can still easily open them in many CAD programs as the AutoCad format is universal today.
That brings me to the single product that I have used since my first Apple ][ in the same form, just updated for the different operating systems, etc., and that is TK Solver by UTS Systems .
TK Solver  is a spreadsheet-like product that lets you enter equations in any order and it will solve for any of the variables in any order. I have used it to design all number of things and still use some of the magnetic templates that I first developed in 1987 (Figure 1).
Figure 1: A TK Solver "Rules" sheet that I did in the 1980s to help with designing pot core inductors. It still works today on Windows 7 just like it did in on my Apple ][ back in the 1980s; in fact the last named version that I have on my current computer of this solver sheet was dated by me back in 1987. Now that's a long and useful life for any piece of software.
Even though the TK Solver "Rules" sheet in Figure 1 was originally written on an Apple ][ computer, it migrated to DOS, then to Windows 3.1 and now to Windows 7 and it still runs like it did on the Apple ][. Naturally I needed to upgrade the program for each of those major shifts, but I did not have to change the templates at all to get them to run on the new PC platforms.
Figure 2: This is the business end of a TK Solver solution. Figure 1 is the "Rules" sheet which determines the equations for the solution. This is the variables page that allows the user to play with various input and output combinations of variables. This particular solution was to find out how much inductance a 1107 pot core with an Al value of 100 can handle given 2.5 amps. The result is about 33 uH, which required 18 turns of AWG 26 wire.
I still use TK Solver on a weekly basis in designing products. It is just so easy to put in equations and then to be able to solve for the variables in any order. TK also has the ability to solve for lists of numbers and plot out the lists, so it is really a "Spreadsheet for Engineers."
What about you - are there any design programs like this out there that you have gotten a long life out of?
 Gimpel Software, www.gimpel.com
 AutoDesk, www.autodesk.com
 UTS Systems, www.uts.com
 Wikepedia page on TK Solver, www.wikipedia.org/wiki/TK_Solver