iGoogle
For a while now Google's iGoogle has some themes to cheer things up a bit. They made a very interesting effort in creating fun and dynamic themes. Weather displaying themes or themes that look different each part of the day or each day of the week.
Custom Themes
Aside from the official Google themes there are alot of usermade themes. Some of them look very impressive! There is also a tool to make it alot easier to use custom themes 
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Below are my favorite themes 
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Classic
Old Design's
Bus Stop (Shows the semi current weather on a pregiven location.)
Dry:
Rain:
Tea House (Shows a different image each part of the day.)
New Design's
JR (Shows a different image each part of the day.)
Solar System (Shows a different image each day of the week.)
Anja Tiger (Shows a different image each part of the day.)
WyriHaximus
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