This article is from the alt-sources-mac FAQ, by Mark Pilgrim (firstname.lastname@example.org) with numerous contributions by others.
sumex-aim.stanford.edu (188.8.131.52) - /info-mac/dev/src
ftp.apple.com (184.108.40.206) - /dts/mac - Apple's primary FTP site. It has
lots of goodies that you won't find anywhere else, sample code and snippets
and tech notes, programming tools and so on.
seeding.apple.com (220.127.116.11) - Not much code here, but it does have the
nic.switch.ch (18.104.22.168) - /software/mac/src - This is a fountain of
source of all kinds, perhaps the most comprehensive source code archive for
the Mac. It has source for lots of unix ported utilities as well as lots of
"normal" mac source code.
ftp.acns.nwu.edu (22.214.171.124) - /pub - Source code for Disinfectant 2.4
(except the anti-virus code) and NewsWatcher.
ftp.qualcomm.com (126.96.36.199) - /mac/eudora - Source code for Eudora.
ftp.ncsa.uiuc.edu (188.8.131.52) - /Mac - lots of NCSA source code
(Telnet, Mosaic etc) and classes for Think Class Library.
redback.cs.uwa.edu.au (184.108.40.206) - /others/PeterLewis - stuff written
by Peter Lewis.
ics.uci.edu (220.127.116.11) - /mac/think-c - Home of the think-c mailing list,
has lots of code & classes, etc., for THINK C.
ftp.cc.umanitoba.ca (18.104.22.168) - /Mac-Develop/Source - Home of the
develop mailing list, also has lots of misc stuff not found elsewhere.