Well one of the best ones turned out to not be a joke. Back in 2013, Adobe launched an open source font called Blank, which renders all Unicode code points using non-spacing and non-marking glyps. (Yes, I swiped that from the repo, I’m not a font guy.)
While very useful, it is basically a font set with 0 width characters. In short, it’s invisible, but it loads really fast.
While the actual launch was a bit before April 1st, the news hit more or less on April Foll’s Day, which led to a mass of chuckles. The confusion was aided and abetted by the line within the codeset that stated, “this font was built using only AFDKO tools”. Several people interpreted that as “April Fool’s Day Knock Out” instead of “Adobe Font Development Kit for OpenType”
So while not an actual joke, it got laughs, and drew a lot of attention to the font that wasn’t there.
One that always stuck with me (thanks to Larry Masinter, who I worked with at Adobe as well).
Meet the Hyper Text Coffee Pot Control Protocol IETF RFC 2324, a (hopefully) still fake protocol for monitoring, controlling and diagnosing coffee pots.
And yes, there is a companion RFC 7168 for test pots,
(I have been corrected, apparently EMACS has a fully functional implementation of HTCPCP via an extension).