1995 (I think, not really sure I'll have to check)

I did a program called MiljoeDialog.
It is still used (actually the comapny that uses it called me up on the 30th of Decemer 1999 and asks me if its Y2K compatible...)

It is used for doing an evironmental 'check' on companies, so they can be certified (ISO..)

It was done in C, and ran on MS-DOS. (they run it in a shell window on Windows today ;)  )