All these other platforms have their own newsgroups or list-servers. If it's not for the Mac, Newton, or PowerMac, it doesn't belong in alt.sources.mac. These can include CDEF's, MDEF's, as well as any other resource. Scripts for either AppleScript, QuicKeys, Userland Frontier, as well as any other scripting system. This includes project files for the Think compilers, as well as Makefiles, AppMaker or Marksman project files, etc.
Source code can be for a full software product, library, patches, glue, or even code snippets.
See below for how to post large archives. Along with the project file, you should include a text file in the archive explaining how to set up the project from scratch. Do not post something that can only be decompressed with a commercial decompression program. Do not post source code to viruses or intentionally malicious software. The alt.sources.mac archive site generates its indices of the newsgroup from the subject lines of the postings in it, so try to make sure that there are relevant keywords in your subject that people can search for when looking for your source code later.