One of the new features in Mac OS X Leopard is the Quick Look. By using the Quick Look function, you can view files without launching the related application to open it.
Most of the common file formats work out of the box with Quick Look. They are typically images, text files, PDF documents, movies, Keynote presentations, and Microsoft Word and Excel files. The best part of Quick Look is that new plugins can be installed to support any additional file formats. The plugins are also used to generate the previews seen in the CoverFlow mode of the Finder.
I always find it strange that on a Mac, the only way to view the content in an archive file (be it zip, rar or any other archive formats), is to un-archive it. There is simply no way to peep into the list of content. Thankfully, with Leopard and the whole Quick Look feature, there is a way to do this: The BetterZip Quick Look Generator.
There are several plugins out there that allows preview of archive files but my personal favourite is this one. With this Quick Look plugin, the content within an archive file can be listed.
The plugin supports quite an array of formats: ZIP, TAR, GZip, SIT, ISO, CHM etc. Apart from listing the content in the archive, it provides useful information like the the folder structure, original size and packed size of each archived item and the file attributes.
Do not limit yourself to just archive files, you can browse through popular repository sites like QuickLook Plugins List and QLPlugins to find a suitable plugin for any file format that you need. The list of plugins are pretty extensive and are constantly being updated. Definitely worth checking them out.