Q: Where and how can I get copies of all these freely distributable programs?
A: As the number of available programs, the number of publicly accessible archive sites, and the number of people trying to access them all grow, this question becomes both easier and more difficult to answer.
There are a number of large, public-spirited archive sites out there, such as ftp.uu.net, archive.umich.edu, oak.oakland.edu, sumex-aim.stanford.edu, and wuarchive.wustl.edu, which have huge amounts of software and other information all freely available. For the FSF's GNU project, the central distribution site is prep.ai.mit.edu. These well-known sites tend to be extremely busy and hard to reach, but there are also numerous ``mirror'' sites which try to spread the load around.
On the connected Internet, the traditional way to retrieve files from an archive site is with anonymous ftp. For those without ftp access, there are also several ftp-by-mail servers in operation. More and more, the world-wide web (WWW) is being used to announce, index, and even transfer large data files. There are probably yet newer access methods, too.
Those are some of the easy parts of the question to answer. The hard part is in the details--this document cannot begin to track or list all of the available archive sites or all of the various ways of accessing them. If you have access to the net at all, you probably have access to more up-to-date information about active sites and useful access methods than this FAQ list does.
The other easy-and-hard aspect of the question, of course, is simply finding which site has what you're looking for. There is a tremendous amount of work going on in this area, and there are probably new indexing services springing up every day. One of the first was ``archie'', and of course there are a number of high-profile commercial net indexing and searching services such as Alta Vista, Excite, and Yahoo.
If you have access to Usenet, see the regular postings in the comp.sources.unix and comp.sources.misc newsgroups, which describe the archiving policies for those groups and how to access their archives, two of which are ftp://gatekeeper.dec.com/pub/usenet/comp.sources.unix/ and ftp://ftp.uu.net/usenet/comp.sources.unix/. The comp.archives newsgroup contains numerous announcements of anonymous ftp availability of various items. Finally, the newsgroup comp.sources.wanted is generally a more appropriate place to post queries for source availability, but check its FAQ list, ``How to find sources,'' before posting there.
See also questions 14.12, 18.9b, 18.13, and 18.15c.