I haven't seen too many, so I'll make a list of what I have found around the web:
The first one I saw was this, which was posted last night. (The person who drew it lives in the UK.) I'm not even in the fandom anymore and it was still pretty neat.
Soon after I noticed that Wikipedia's front page had changed. Here's a screenshot I just took; it's most likely a bit different from what it was last night, but it'll have to do. My favorite bit is "A newspaper discovers that pay-per-view porn is amongst a number of unusual things being purchased by British MPs on their claimed expenses." I heard that story on BBC just a day or two ago!
Now for what I've actually seen today. As far as I've found, dA hasn't done much for April Fool's this year; I've only spotted cute little snowman symbols (☃) replacing the equal signs that are next to beta testers' names. No Mudkips this year, unfortunately. I thought that was awesome.
Soon after I came home, I found that a certain video was posted on capslock_mgs:
(Well, okay, the link to where the video is was posted, but the site wouldn't buffer for me so I had to look for it on YouTube.)
Oh yeah, and Photobucket is overrun with garden gnomes.
Edit @ 3:54 PM: Oh yeah, I forgot about what happened to the Google image search! It's... A lot different than it was earlier.
Edit #2 @ 4:18 PM: Ohoho, well played. (Warning, a bit NSFW.)