If you have a very old Gentoo system that you want to upgrade, you might have some issues with too old software and Portage which can't just upgrade to a recent state. Although many methods exist to work around it, one that I have found to be very useful is to have access to old Portage snapshots. It often allows the administrator to upgrade the system in stages (say in 6-months blocks), perhaps not the entire world but at least the system set.
Finding old snapshots might be difficult though, so at one point I decided to create a list of old snapshots, two months apart, together with the GPG signature (so people can verify that the snapshot was not tampered with by me in an attempt to create a Gentoo botnet). I haven't needed it in a while anymore, but I still try to update the list every two months, which I just did with the snapshot of January 20th this year.
I hope it at least helps a few other admins out there.