The Simpsons is famous for its stellar roster of special guests. Once the show ascended into the upper echelon of pop culture phenomena in the early '90s, landing a guest spot on The Simpsons became one of the most coveted television appearances in Hollywood. As Springfield's favorite family began to grow in renown, The Simpsons even began to lend their likeness to endorse a plethora of products. However, despite the staggering number of special guests they've featured or commercials they've appeared in, The Simpsons rarely cross over with other television shows.
Outside of The Critic, Family Guy, The X-Files, 24, Futurama, and a handful of others, The Simpsons have pretty much played in their own sandbox over the years. This is why the video linked above from an episode of L.A. Law featuring Dan Castellaneta as the voice of a Homer Simpson mascot is such a unique relic from the 90s.
