Though there are a number of ZIP utilities out there, and OS X natively supports the ZIP format, it requires that the ZIP file be extracted completely before it can be viewed and it has problems on older ZIP file formats. David Alison talks about how Zipeg, a free utility, makes this an easy task.
Zipeg also supports ZIP, RAR, ARJ, LHA/LZH, 7z, TAR, GZ, TGZ, BZIP2, CPIO, RPM, ISO, CHM, Z, CBR, CBZ, WAR, EAR... lots of formats.