First and foremost, they changed their name to "Topeka" for the day (which is hilarious if you happen to recall that Topeka changed their name to Google for the month of March).
Gmail was missing vowels--low key, but well played.
Google UK offered an Android app that will translate animal noises into English.
But my favorite was the Google Annotations Gallery (GAG) from Google Code. It's a set of annotations you can use to express yourself in your code. There's the "Disclaimer" package, which includes such gems as @AnimalsHarmedDuringTheMaking or literary tags where you can label your method by its meter or as a Haiku. Or you can annotate with exclamations like @LOL, @Facepalm or @WTF.
That makes for a great April Fool's joke (and read the quick-start guide, if you're the least bit code-savvy it's hilarious). But the best part is, it's not a joke. They made it for real. You can put it in your classpath and use those annotations. You can mark your own code with @Hack if you want to.
And this is why Google is the awesome. Other reasons as well.
Have a good weekend.