Are different version naming conventions suited to different projects? What do you use and why?

Personally, I prefer a build number in hexadecimal (e.g 11BCF), this should be incremented very regularly. And then for customers a simple 3 digit version number, i.e. 1.1.3.

1.2.3 (11BCF) <- Build number, should correspond with a revision in source control
^ ^ ^
| | |
| | +--- Minor bugs, spelling mistakes, etc.
| +----- Minor features, major bug fixes, etc.
+------- Major version, UX changes, file format changes, etc.


I tend to follow Jeff Atwood's opinion of the .NET convention of version numbering.

(Major version).(Minor version).(Revision number).(Build number)

More often than not, for personal projects, I find this to be overkill. The few times where I have worked on substantial projects like search engines in C# I've stuck to this convention and have been able to use it as an internal tracker effectively.
