So far, the best April fools have been hanging out at Google, which today brings us things like gDay ("Using a mashup of numerous factors such as recurrence plots, fuzzy measure analysis, online betting odds and the weather forecast from the iGoogle weather gadget, we can create a sophisticated model of what the internet will look like 24 hours from now"); and Project Virgle ("starting in 2014, Virgin founder Richard Branson and Google co-founders Larry Page and Sergey Brin will be leading hundreds of users on one of the grandest adventures in human history: Project Virgle, the first permanent human colony on Mars."). The Virgle pioneer application alone is a hoot.
Any other good pranks out there?
Update: Wired has some good ones in its top 10 list. Like this one:
Which reminds me of this great one from the BBC: Flying penguins
OK, back to work!