It used to be the case that you could install packages on Mac OS X by using lsbom to inspect the list of installed files recorded in the files under /Library/Receipts.

that location  /private/var/db/receipts.

At some point — I’m not sure which — Apple added an easier-to-use solution in the form of pkgutil.

pkgutil --pkgs # list all installed packages
pkgutil --files the-package-name.pkg # list installed files

After visually inspecting the list of files you can do something like:

pkgutil --pkg-info the-package-name.pkg # check the location
cd / # assuming the package is rooted at /...
pkgutil --only-files --files the-package-name.pkg | tr '\n' '\0' | xargs -n 1 -0 sudo rm -i
pkgutil --only-dirs --files the-package-name.pkg | tr '\n' '\0' | xargs -n 1 -0 sudo rm -ir

Needless to say, extreme care should always be taken when removing files with root privileges. Particularly, be aware that some packages may update shared system components, so uninstalling them can actually break your system by removing a necessary component.

For smaller packages it is probably safer to just manually remove the files after visually inspecting the package file listing.

Apparently, there was once an --unlink option available in pkgutil, but as of Lion it is not mentioned in the man page. Perhaps it was removed because it was deemed too dangerous.

Once you’ve uninstalled the files, you can remove the receipt with:

sudo pkgutil --forget the-package-name.pkg

For more details, see the pkgutil man page.