Episodes of The Simpsons that have other characters from other shows "cross over" to the show like "The Springfield Files," where Mulder and Scully from The X-Files made an appearance and "A Star is Burns," where, much to Matt Groening's annoyance, Jay Sherman from The Critic visited Springfield.

Note: This category only contains episodes where the crossover is essential to the plot, and not counting brief cameo crossovers such as King of the Hill's Hank Hill appearing in Bart Star. 

The Springfield Files

24 Minutes

A Star is Burns

Mother Simpson

Judge Me Tender

The Computer Wore Menace Shoes