I'm not talking about software
with big version number
s like 5.0. I'm talking about software with version numbers so big that they are either very inflate
d or a part of history
Fractint, an old piece of software that was originally made to draw fractals for MS-DOS, has been updated so much that it is at version 20.0. It is being supplemented and often replaced by newer programs, and it is fast becoming historical. Its documentation talks about being developed by the Stone Soup Group on Compuserve, which gives you some idea how old it is.
But that is hardly anything compared with emacs. No matter what you think of the text editor/email client/everything else, (I'm not going to get into that flame war), there is one fact: it is very old, and has been picked over by thousands, been ported to more systems that I can count on my hands without counting in binary, had many spinoffs made, and is at version 21.2 last time I checked. And that's just GNU emacs, which doesn't include all the other versions. You might even call it a deflated version number.
Big version numbers also imply that the software, for better or for worse, survived for a long time (with a few exceptions, which I'm not talking about here for the sake of argument, such as X11). There are plenty of pieces of software that didn't survive long enough to get a version number of this magnitude. Hopefully this will lead to Darwinian selection among software, but I'm not getting my hopes up too far.
It has been pointed out that LaTeX has a particularly bizarre version numbering system, which I think is a good idea for something as mature as LaTeX -- the version numbers tend towards pi. The first version was called 3, then 3.1, then 3.14, and as it gets better (that is, closer to perfection) it gets closer to pi. So I suppose that the analog of big version numbers with the LaTeX system is very long version numbers. Weird, but cool.
Ameoba has also said that the emacs version number has been inflated. Still, it seems like a good reflection of reality, considering how many people have picked at it and added things to it.