Old versions of Oracle... don't forget about Oracle EDelivery
I was recently looking for old (not ancient, just old) install media to support a database migration. The usual collection of download links on OTN usually just has the last 2 releases. If you need older versions (in this case 9i) then hit up the EDelivery site and you should be able to find them.
If you need a much older version (like 8i or gasp! 7) I have heard you may be able to obtain it through an SR if you have a current support contract but luckily I have not had to test that route.