According to YSlow, a number of JS and CSS files on commons aren't served gzip'd. The list includes:
(27.4K) http://commons.wikimedia.org/skins-1.5/common/wikibits.js?165
(5.3K) http://commons.wikimedia.org/w/extensions/CategoryTree/CategoryTree.js?3
(4.7K) http://commons.wikimedia.org/skins-1.5/common/ajax.js?165
(4.5K) http://commons.wikimedia.org/skins-1.5/common/ajaxwatch.js?165
(23.0K) http://commons.wikimedia.org/skins-1.5/common/mwsuggest.js?165
(23.6K) http://commons.wikimedia.org/w/extensions/OggHandler/OggPlayer.js?8
(1.2K) http://commons.wikimedia.org/w/extensions/CategoryTree/CategoryTree.css?3
(4.7K) http://commons.wikimedia.org/skins-1.5/common/shared.css?165
(4.8K) http://commons.wikimedia.org/skins-1.5/common/commonPrint.css?165
(27.3K) http://commons.wikimedia.org/skins-1.5/monobook/main.css?165
(8.4K) http://commons.wikimedia.org/skins-1.5/chick/main.css?165
Version: unspecified
Severity: enhancement
URL: http://commons.wikimedia.org/wiki/Main_Page