(La)TeX. A few weeks ago I re-typeset a 10 year old document in the latest version of LaTeX. Came out just right. I suspect that if I had a ten year old version of LaTeX2e I could typeset a modern document.
...and we shall see if OO formats 10 years from now will open in today's version of OO, or the other way around.
This is why I use LaTeX for more things than I probably should. On the other hand, I can still read and typeset those text files, and on whatever my platform of the day is. There simply isn't anything else that delivers the typesetting quality and longevity.