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).