In my case, it is the GNU Chess engine with the XBoard interface. It was available on the first Linux distro I ever installed (Red Hat Linux) and chess being my favourite game it has always been installed on every laptop I use personally since. GNU Chess has been ported to Windows too. It is reasonably strong and a good opponent for budding chess players. GNU Chess is open source and free.
GNU Chess Version 1 was written by Stuart Cracraft, in 1984 (pre-GNU era) and is one of the earliest pieces of software in the GNU package. It has been maintained and developed by many other programmers since.
Leaving aside operating systems and frameworks, what is the oldest and maintained software you are still in love with?