Homebrew is awesome. If you are a developer, and use a Mac in your daily adventures, you should install it. Period.
This little tip might be handy, specially when you want to rollback to a previous version of a package after an update. Because, you know, shit happens, and more often than not, updates break stuff.
You can list all the versions of a package installed in your system:
$ brew info mongodb
And to force the system to use a particular version:
$brew switch mongodb 2.6.8