TeX Versioning System

The TeX (the basis for LaTeX) typesetting program has a pretty unique versioning system: Approximation of pi. So the first version is numbered 3, the next 3.1, next one 3.14, ad infinitum. Well, not ad infinitum since 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 "features".