Thanks to Rich La Bonte <firstname.lastname@example.org> for suggestions concerning this section.
It is always a good idea to compress a module before you distribute it. Especially when using telecommunications systems (i.e. telephone lines or networks), data compression can reduce a lot of transmission time and bandwidth. The two main archivers used in compressing mods are LHA/LHarc and PKZIP. LHA is used mainly on Amigas, but compatible archivers/dearchivers exist on just about any platform. The same goes for the MS-DOS originated PKZIP. Other archivers may perform better compression, but aren't a good choice if you want your file to be decodable by as many people as possible.
Note though that even though LHA/LHarc's archiving is generally compatible across platforms, users should be aware of the limitations of other systems. While flipped orders of filename and extension is rather easy to fix and is not a problem (by which I want to express that I discourage discussions about efficiencies and deficiencies of the various platforms), there are several incompatibilities which hinder distribution.
LHA/LHarc for Amiga can store attributes that prevent MS-DOS LHA from dearchiving the files contained. A filename beginning with a period has the same effect, as MS-DOS uses the period to symbolize the current directory only. Mac files include a header containing a resource and a data fork. The resource fork is absolutely Mac-specific and is incompatible with any other system (AFAIK).
UPDATE NOTE: Netiquette still ensures that most Usenet postings are compressed in some form. LHA, ZIP, ARJ, SIT are the predominant varieties but you will find others around (RAR for example is fairly new). Most decent MOD sites will give you options on where to download compression tools from but just in case you want it *NOW*!! Do a web search using any of the archive extensions and 'compression(ie .zip, .lha, .rar compression)' and you should find the program quickly enough.
NOTE: An Iomega 'Zip' drive and a PKZIP file are very different methods of file compression. The so called 'ZIP Drive' is a peice of hardware, PKzip and PKunZip are software from PKware: http://www.pkware.com/