First up, this special preview of the new season of Doctor Who appeared online on April 1st.
Locus Magazine had their usual selection of April Fools news items as well including the first season of Top Writer on Bravo and Cory Doctorow releasing himself under the Creative Commons license. No direct link to the stories, but look at all of the April 1st stories.
George R.R. Martin also had a post about a solution to how to end The Song of Ice and Fire series by a crossover with the Wild Cards universe!
TrekMovie.com has a roundup of some of the various Star Trek related April Fools jokes from this year.