This fall The Simpsons is going to have its first big crossover in almost 20 years, when the cast of Family Guy visits Springfield. It's going to be the second time ever in which characters from other animation series appear in full capacity in the longest running sitcom in American TV history.
The first crossover was with The Critic, in the episode A Star Is Burns, broadcasted on March 5, 1995. While The Critic was great, it definitely didn't have the same star status as the Griffins.
The other incoming one-hour-long crossover will be the much awaited appearance of the full Futurama cast in an episode called Simpsorama, which will be broadcasted in November.