Every time I log into a character, I hear a random snippet of boss dialog; so far I've had Algalon, Kil'Jadeen, Rhyolith and XT-002. Apparently it's from DBM, which means Blizz would have been out-April-Fooled by an addon.

LMBO!
Hey, me, here's a thought - How about checking Blizzard's main page or facebook page!
They have 2 new things up, only one of which is the clear joke. Guess which one!

There's a couple of parody minigames on the Starcraft site, and Bioware's Space WoW: Same Game, Worse Engine site has a preview of some new gameplay for the Ship Droids.

It's pretty off-topic, but the best April Fools gag I've come across this year has actually been the Single Player Commands mod on Minecraft. Logged in and a message popped up from the mod that said "Try out our newest command, /roll." I typed it in thinking something to the effect of rolling forward would happen, or maybe even the staff roll for the mod would pop up.

I was wrong. "Never Gonna Give You Up" started playing through Minecraft. And I laughed. Hard. Literally for a minute solid. Best laugh I've had in ages.

@ Andrew: I thought I was becoming psychotic. Thank you for clearing this up.

Crabby is back on the world of warcraft page and if you click on him he will put on different hats.

@Andrew It's not just when you log in either. I heard Medivh in the middle of questing.

Yay for Crabby!

I can't believe it, but I actually like him! I'm glad he returned for another April Fool's Day.
(As long as he doesn't stay too long and Rebecca Black roll me >.>;; )

OMG the supply depot game credits go on forever. They are still going. Like 5 minute now.