The Unarchiver is an Objective-C application for uncompressing archive files.

• Supports more formats than I can remember. Zip, Tar, Gzip, Bzip2, 7-Zip, Rar, LhA, StuffIt, several old Amiga file and disk archives, CAB, LZX, stuff I don't even know what it is.
• Copies the Finder file-copying/moving/deleting interface for its interface.
• Uses the character set autodetection code from Mozilla to auto-detect the encoding of the filenames in the archives.
• Supports split archives for certain formats, like RAR.
• Version 2.0 uses an archive-handling library built largely from scratch in Objective-C, which makes adding support for new formats and algorithms very easy.
• Uses libxad (http://sourceforge.net/projects/libxad/) for older and more obscure formats. This is an old Amiga library for handling unpacking of archives.

Work is being done on making at least the unarchiving engine itself multi-platform.