Windows

design.varos
Features the Doctor Who series, and Godzilla, along with a few other themes.
http://www.varos.net/design/themes/

DeskFlag
Places an animated American flag on the desktop.
http://www.deskflag.com/

DesktopDecor
Categories include wild life, scenic, Disney, and sports. Also offers other desktop decorations.
http://www.desktopdecor.net/

The Freshman Mafia
Offers some Windows Vista and XP visual styles, as well as an Age of Empires 3 Dracalord scenario set.
http://www.freshmanmafia.webs.com

Iponsel.com
Database of free Windows Mobile themes and pocket PC themes.
http://iponsel.com/

LightStar Technologies
Custom Windows XP and Vista themes. Corporate branding and GUI customization services available.
http://www.lightstar1.com/

Themes 4 Vista
Offers several Windows Vista themes, gadgets, icons, and wallpapers. Themes can also be used with Windows XP.
http://www.themes4vista.com

Trish's Theme Terrain
A collection with many different categories including artwork, God Bless the USA, machines, and people.
http://www.trishsthemes.com

US Auto Parts.net
BMW automobiles including the E46 3 series convertible and the M3.
http://www.bmwworld.com/media/themes/

Vista Desktop Themes
Includes wallpapers, tweaks and gadgets. Allows notifications of updates via RSS.
http://vistadesktopthemes.com

Vista Themes
Windows Vista/XP themes, icons, skins, screen savers and wallpapers. Includes holidays and abstracts.
http://www.vista-themes.net/

Vista Themes
A source for Windows Vista themes, wallpapers and icons. Includes how-to articles.
http://www.myvistathemes.com

Vista Themes
Includes themes, wallpapers and icons. Also contains tips and tricks for Vista.
http://www.9vistathemes.com

Windows Mobile Themes
Database of themes for version 6. Includes animals, cars, cartoons, movies, space, nature, and buildings.
http://www.wm6themes.com/

Windows Vista Themes
Free Windows XP and Vista themes, icons, gadgets, wallpapers, and howto's.
http://www.wvistathemes.com/

Windows XP Desktop Enhancements
Contains Windows XP themes, skins, and screensavers. Also includes desktop images.
http://www.microsoft.com/windowsxp/downloads/desktop/default.mspx

XP Themes
Themes include Halloween, Christmas, Thanksgiving, and New Years.
http://www.appleblossomart.com/xpthemes.htm

XPTheme
Windows XP themes, wallpapers, boot screens, logon screens, and screensavers.
http://www.xptheme.info/