Now a .pkg bundle essentially contains an Archive.pax file, which is an archive of the files to be installed, so another solution is to use pax in the Terminal. But the man page for pax is not exactly enlightening; I just want to quickly decompress a pax file, for heaven's sake!
But as is often the case, the obvious solution is the last one you would think about. Simply copy the Archive.pax.gz file out of the package bundle and double click it in the Finder! The Finder then happily decompresses everything in a folder, and you can install everything manually.
[robg adds: This older hint talked about using the Terminal to peek inside packages; with the changes in OS X since it was written, it's now possible to do this sort of thing from the Finder.]

