Quicklook was for me one of the best improvements in Leopard. Even better that it is expandable with plugins and that XCode comes already with a handy project template. See  www.quicklookplugins.com  for a list of available plugins. One thing thats really missing is that you dont get a hexviewer for files OS-X does not know.

The basic idea was to catch all file types as described in  the apple documentation  by catching public.data.

After that the plugin simple reads in the first 4k of the file and renders them in a webview with some jquery to provide for tabs, highlighting, ...

Just copy the .qlgenerator directory into HOME/Library/Quicklook.

The sources are located at github.