I really liked collegehumor.com's joke.
instead of loading their homepage it goes to a rather official looking government site for the Department of Defense.
"Attention: Your IP address has been logged in relation to an ongoing investigation.
This website has been shut down due to Anti-American content and threatening anti-government material. The site and all of its visitors are being sought in relation to an investigation regarding various posts and publications of an illegal nature. We will retain your IP address so that you may be contacted about your involvement with this site.
thank you"
LOL it was something like that
after about 10 seconds it goes to their home page "APRIL FOOL!!"