No downloads or registration is needed. Just upload your picture, select the desired effect from the list, and it is done. Now, you can simply download the resulting funny image to your Hard Drive. There are more than 50 very different effects to choose from. An additional use for the free service is making a nice striking avatar for your forums or social sites profiles.
Web Site access: http://www.photofunia.com/
Update 12-27-13: As of today, Photofunia offers almost 400 hundreds effects to help you to make a lot of fun with your pictures the way you like.