I have yet to think of any really good, yet safe (and safe-for-work) pranks and or hoaxes of my own, so I'll share two of my favorites -- both of which started with the BBC.
The first, of course is classic. The video below is a bit fuzzy, but a better one can be seen here.
Another good BBC hoax, narrated by Monty Python member Terry Jones, features Adélie penguins that have learned how to fly -- and migrate.
Wikipedia articles: April Fool's Day, Spaghetti Trees and Flying Penguins.
Youtube clips of the spaghetti harvest, available in fuzzy or not fuzzy
Youtube clip of the flying penguins