Below are some image taken from various April fools jokes NG played:

2007:
All admin & user online record.
NG 2.0 - pic by, kes.

These 3 images were of a failed fooling. I say failed as they were only live (on the net) for maybe 5 minutes. Fortunately, I was there with my finger on the PrtScrn button!

NG Index
News link: 1
News link: 2

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2008:
Truly a shity fools (pun half intended).
All this year had was just 2 image swaps. The main logo and site background... Not even an all user admin orgy took place, something that should happen every year...
Logo 1
Logo 2
Logo 3
Logo 4

Site BG
Lets hope 2009 will have a better joke.

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And they did!
Chinagrounds 2009:

See: Chinagrounds