The best April Fools joke I can remember was an anti-joke, when Google announced Gmail on the 1st of April. They were promising what were then unbelievably generous allowances and no one could agree on whether they were serious or not.
https://web.archive.org/web/20100718195724/http://www.perl.c...
The official Perl.com site announced Perl and Python were going to merge so you could run both languages in the same interpreter. In itself that was quite amusing, but the best part is that the Raku project actually adopted the name for their VM because it could actually do that, and became https://en.wikipedia.org/wiki/Parrot_virtual_machine, so what started out as a proper joke turned out to be real.