I don't think if I know of a single game that has LIMITLESS playability. I mean, sooner or later, I will get bored of any game and need to break for weeks, months, years, and then come back to it later. However, a few examples of games that I still can pick up and play anytime and still do to this day, I will leave out any that eventually died, even if they did offer many years.
In no particular order...
1. Doom
2. Starcraft: Brood War
3. Street Fighter 3: 3rd Strike
Hmm, I know I am missing something but I can't think of any others I still play often enough to count. Sure, I still pick up old games all the time, but not often enough to match your criteria.