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.