Even though the graphic the site creates looks like a real tweet, it's nothing of the sort. Once you enter a Twitter handle, TweetForger simply scrapes up your victim's twitter background and then re-creates it almost perfectly with your chosen text inserted within.
While this looks pretty scary -- and somebody might be calling up some lawyers any minute now -- it's just harmless fun. Here's how the creators of Tweetforger explain it:
"TweetForger is a fun way to make it appear as if a Twitter user has tweeted something. You can pick any user you want, enter some text, and TweetForger will create an individual, look-alike tweet. Nothing you do here will appear in anyone's Twitter stream, even your own. TweetForger is just for fun. Please don't be a jerk."
There's still a few hours left of April Fools' Day, so if it appeals to you, go ahead and make fools out of all your friends.
Meanwhile, here's an example of TweetForger's handiwork:
