Mac OS

GirlieMac
Free Kaleidoscope schemes, wallpaper in resolutions 800x600 and 1024x768 and icons.
http://girliemac.com/

The Kaleidoscope Scheme Archive
Resources for Kaleidoscope users including third party schemes and tools.
http://www.kaleidoscope.net/schemes/

Mac Themes Project
Designing information and downloads, including themes for Mac OS X.
http://sourceforge.net/projects/macthemes/

MacThemes 2.0
A resource for OS X themes, icons, and desktops. Features weekly updates, theme reviews and insider interviews.
http://www.macthemes2.net/

Max Themes
For Macintosh OS X. Supports versions 10.2, 10.3, and 10.4.
http://www.maxthemes.com/