It was only one year ago that I realized that various sites like to pull April Fools’ pranks.  Here are some of the pranks I encountered in the sites I frequent:

Ubuntu Forums changed up its color scheme and added an “OMG pinK pOnieS” forum.

Fark.com had their server hacked by hamsters:

Woot.com had a fake Woot-off in which they listed many instances of a device that can give you 120 V at 20 W on a battery of sorts.  The detailed image (which can be different from what is shown on the front page) changed accordingly.  Here is one of them:

You can see a list of other pranks here: http://en.wikipedia.org/wiki/April_1%2C_2008