StuffIt

The defacto compression algorithm for the Macintosh (or a request of a person you don't like). Put out by Aladdin Systems, this age-old program handles all of one's Mac compression and decompression needs. It also handles the resource fork that is unique to the Macintosh file system.

It's one of the Macintosh's killer apps, and it's free to download the expander. It's a necessary utility for any macintosh these days. It is comparable to the zip format on the PC, and all of one's Internet downloads are in this fashion.

I still think it is also a compressed version of a request, of the form: "Just take that file and go... "

Stuffit is definitely a killer app, but most of what JayBonci mentions is solely the StuffIt expander / compressor. This application acts similar to Winzip and Winrar except that it handles SO many more file types. StuffIt is able to efficiently work with the following:

Apple Single
Arc
Arj (works on all platforms but Macintosh)
Binhex
BtoAText
Bzip2
CompactPro (works only on Macintosh)
UnixCompress
GZip
Lha
MacBinary
MIME
PrivateFile
RAR
Self-extracting archives (MAC or Windows)
Stuffit Segments (like rar partials... sorta)
Shrinkwrap & DiscCopy images (Mac only)
StuffIt file format (.SIT)
Stuffit X (.SITX)
Tar
UU
Zip

The stuffit file type and stuffit expander works for any system (mac, win, unix) and it's compression ratio is greater than both rar and zip in most situations. Aladin (the company that produces Stuffit) claims Compression as high as 98% can be acchieved using StuffIt (ie a 96 meg file into a 2.7 meg file) and can be done QUICKLY (compression mentioned before takes only 6 minutes as opposed to winzip which would take 13). Aside from being extremely compatible, StuffIt is highly secure offering 512 bit encryption of files. Yay privacy! Lastly, as mentioned above this information simply refers to the Stuffit Expander. There are also applications like the Stuffit Joiner (combines, for example, a rar file split into 15 parts back into one file) and an image mounter (reads bin, img, and other image file types).