RAR

RAR (Roshal ARchiver) is a file archive format created by Eugene Roshal that utilizes a propriety data compression algorithm that offers superior compression over most other formats. RAR utilizes similar compression methods of other formats, except it has a special function called Solid compression which compresses files together instead of individually like the more common Zip format does.

RAR supports other features like multi volume archiving, Unicode file name support, and encryption.

Links
rarlab.com - RAR Lab's official site.