In response to an article about Archive Utility's preferences, a reader asked if there was a way to show the contents of an archived file. I used to have a QuickLook plugin for this, but it stopped working at some point and I had never gotten around to finding a new one.
The BetterZip Quick Look generator works fine for me under Snow Leopard, and supports QuickLook for the contents of a huge number of various archives. According to the website, it supports ZIP, TAR, GZip, BZip2, ARJ, LZH, ISO, CHM, CAB, CPIO, RAR, 7-Zip, DEB, RPM, StuffIt's SIT, DiskDoubler, BinHex, and MacBinary. Or, to put it another way, it supports all the formats you are ever likely to come across, plus about 5 more.
Simply download the file (from the page above), and install it either to /Library/QuickLook/ (if you want the plugin to work for anyone who logs into the machine) or ~/Library/QuickLook/ if you want it to just work for the current user. If you have never installed a QuickLook plugin before, you may need to create that folder. Note that although the feature is called "Quick Look" the folder name should not have a space in it.
Then you need to restart the Quick Look framework. This either requires waiting (if you're the patient kind), logging out/rebooting, restarting the Finder (according to the website, but that didn't work for me), or open Terminal and type
which is what I did, and it started to work immediately (you should see a message "qlmanage: resetting quicklookd" after you enter the command).
BetterZip QuickLook Generator is a free plugin by the folks at MacItBetter.com who also make a $20 program called BetterZip which gives you a whole host of options for creating and managing archives. I haven't used it myself, but it looks pretty impressive if you want some advanced options for archives. Our own Brett Terpstra reviewed it back in May if you'd like more information. Dave Caolo also mentioned BetterZip Quick Look Generator in his 2008 article 10 ways to get the most out of Quick Look. If you want to learn more about additional Quick Look plugins, that's a good place to start.