"TeX has an idiosyncratic version numbering system. Since version 3, updates have been indicated by adding an extra digit at the end, so that the version number asymptotically approaches π. The current version is 3.141592. This is a reflection of the fact that TeX is now very stable, and only minor updates are anticipated. TeX developer Donald Knuth has stated that the "absolutely final change (to be made after my death)" will be to change the version number to π, at which point all remaining bugs will become permanent features.

In a similar way, the version number of METAFONT asymptotically approaches e."

From http://en.wikipedia.org/wiki/Software_versioning#Software_versioning_schemes