The
Posix-correct place to put
third-party software is now
/opt.
The company I work for has traditionally used
/usr/local for the *operational* versions of software developed
in-house.
This includes binaries as well as
shell scripts, and the original versions of
OS scripts we have had to customize. Oh, and of course documentation.
It makes a lot of sense when you're frequently backing up and restoring stuff, or updating the OS. You know where all your weird stuff is.