Usually I try to access the Wayback Machine version of of a developer's/publisher's website, if this is known (MobyGames often gives good leads on that). If the site's archived pages are functional the download links often point to an FTP, even if some of the files are mirrored as HTML at the same site. The Links section is also worth checking, as it might give leads to other interesting sites.
You can also approach this from the opposite end so to speak, if you're looking for a specific file. Old messages in newsgroups (archived by Google Groups) or stored search results at places like FileWatcher (and FTP search engine) can give clues to possible download locations. Just Googling for an exact files name (by enclosing it in quotes) may give some interesting results. Also there are pages with links to original downloads, like this
, or the archived pages of Lord Soth's Games on the Internet
You can also surf Wayback Machine copies of gaming magazines, e.g. PC World, GamesMania, NEXT Generation etc. (some of them are mentioned in this thread). In some cases this yields files (as described here
), in others - info on file names and sizes. If you use this kind of combined approach to sources, you may find lots of interesting stuff, although it's not always what you were originally looking for