I've been using "Tetris" for, like, ever.

I went through the first 125 pages of the most used iGoogle themes and came up with the following (in alphabetical order) as my favourites:

Tory Burch

JR

Todd Oldham

Tetris

The rest were mostly a mix of bad art, bad layout, and bad palettes, with a relative few solid designs that just didn't really appeal to me.