Update: we just found another one - Google TiSP. Actually, this one is semi-plausible. Now we gotta beware of the double bluff. ;)
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It's only just turned April Fools, but already the gags are piling into our inbox. Google is out the door running, launching GMail Paper today - send your email via snail mail, accompanied by targeted ads. No doubt it'll be called Google Mail Paper in the UK and other markets where the trademark is disputed.
Looks like Yahoo Underground will also turn out to be a prank (we hope).
The problem we face is that it's pretty much impossible to tell which of the submissions today are jokes, and which ones are real. Take a look at WhoIsSick, for instance, a Google Maps mashup that helps you to find sick people in your area - fact or fool?