Homicide: Life on the Street and Law & Order
Even though Law & Order is still on the air, we felt this crossover was too good to leave off the list. The worlds of Homicide and Law & Order, both of which aired on NBC, intersected several times, but our favorite crossover episode occurred in the February 7, 1996 Law & Order episode "Charm City," and companion Homicide episode "For God and Country" on February 9. When a deadly subway bomb in New York City was traced to a white supremacist who bombed a church five years earlier in Baltimore, Detectives Munch (Richard Belzer) and Pembleton (Andre Braugher) from Homicide high-tailed it up to New York City to stake their claim on the case in the first episode, and then Law & Order's Detectives Briscoe (the late, great Jerry Orbach) and Curtis (Benjamin Bratt) headed down to Baltimore to wrap up the case in the second. The territorial sparks flew between both sets of cops, and we even found out that Briscoe had slept with Munch's ex-wife -- all in all, it was a satisfying two-parter that made sense in the worlds of both shows. Crossover sidenote: J.K. Simmons, who played the white supremacist, went on to become the recurring Dr. Emil Skoda, who first appeared on Law & Order in 1997. And Richard Belzer's John Munch, who is actually the most crossed-over character in TV history having appeared on nine different shows, moved from Baltimore to join the New Yorkers of Law & Order: SVU after the demise of Homicide in 1999.

Buffy the Vampire Slayer and Angel
Perhaps the most complete spin-off-crossover phenomenon happened between creator Joss Whedon's Buffy the Vampire Slayer and Angel, both of which aired on The WB. Guilt-ridden, soul-possessing vampire Angel (David Boreanaz) appeared in the very first episode of Buffy on March 10, 1997, and became a series regular as his romance with the titular Buffy Summers (Sarah Michelle Gellar) heated up. But in the season 3 finale, Angel moved to L.A. in order to continue his quest for atonement on his own -- thus spinning off into Angel, which premiered on October 5, 1999. The crossovers between these shows are too many to count -- almost every major character from Buffy ended up making multiple appearances in Angel and Angel himself occasionally returned to Sunnydale when plotlines crossed between the series. The sheer number of connections and intersections causes the shows to be viewed as part of the same basic "Buffyverse" by fans. Buffy ended in 2003 and Angel followed in 2004, but the two series live on in the hearts and minds of fans all over the world.

You can watch one of Buffy's appearances on Angel from the Season 1 episode "Sanctuary" here:

Ally McBeal and The Practice
Both shows were about lawyers, both were set in Boston, both were from creator David E. Kelley -- a crossover was practically inevitable, despite that fact that Ally McBeal was a legal comedy that aired on FOX and The Practice was a dark legal drama that aired on ABC. The crossover began on April 27, 1998 on Ally McBeal in the episode "The Inmates" when one of Ally's clients, accused of murdering her husband with an ax, claimed that she was Lizzy Borden in a past life and the McBeal-ers had to call in the more experienced lawyers from The Practice to help with the case. But Bobby Donnell (Dylan McDermott) ended up advising the client that Ally's (Calista Flockhart) firm wasn't up to the job and stealing her away to The Practice in the episode "Axe Murderer," which aired on ABC immediately after the Ally episode aired on FOX. Ally and Bobby had a bit of a love connection, but all went to shreds after the main suspect shot himself in the courtroom and Ally returned to Cage, Fish and Associates. But the crossover continued a few weeks later in Ally McBeal's season finale "These Are the Days," when Bobby returned to ask Ally for her help with a case involving a heart transplant between best friends. Though the two got closer and closer over the course of the episode, in the end they decided that there was no way their relationship could work, thus ending the possibility of an intra-show relationship.

The Practice and Boston Legal
Despite the fact that one David E. Kelly crossover already made our list, this unique crossover-spin-off-conflagration deserves a spot for sheer cleverness on the part of its creator. In 2003, The Practice had finished its seventh season on ABC and was flagging in the ratings -- so Kelley brought in James Spader to play Alan Shore, a scruple-free attorney who was hated by his colleagues but would win cases by any means necessary for deserving clients. Alan was eventually fired for misconduct and ending up suing Young, Frutt, & Berluti -- and who did he hire as his lawyer but the unforgettably eccentric Denny Crane (William Shatner)? This all happened with just five episodes left in the final season, at which point Shore decided to join with Crane's law firm and thus the spin-off Boston Legal was born, which then premiered on ABC on October 4, 2004. And so the world of The Practice continued after its demise, although Boston Legal aired its final episode on December 8, 2008.

Mad About You and The Dick Van Dyke Show
Even if this crossover didn't combine two of the smartest shows on television, it would be notable merely for involving a show that had been off the air for twenty-nine years at the time of the crossover episode. "The Alan Brady Show" aired as an episode of Mad About You on February 16, 1995 and involved Paul Buchman (Paul Reiser) asking Alan Brady (Carl Reiner), the star of the show written by Dick Van Dyke's character thirty years earlier on The Dick Van Dyke Show, to narrate a documentary on the history of television. The episode was full of references to The Dick Van Dyke Show, and was made even more heartwarming by the fact that Carl Reiner was Paul Reiser's real-life hero. Though it only lasted one episode, this crossover is surely one of the most satisfying there is.

The Simpsons and The X-Files
A wacky cartoon about the misadventures of a family that happens to be yellow and a spooky sci-fi series about proving the existence of extraterrestrial life might seem to have nothing to do with each other, but on January 12, 1997 an episode of The Simpsons called "The Springfield Files" brought these two discordant worlds together. Homer is walking home one night after a binge session at Moe's Tavern when he comes across a glowing, alien-shaped being in the woods -- so of course Fox Mulder (David Duchovny) and Dana Scully (Gillian Anderson) from The X-Files arrive to solve the case. Though The Simpsons constantly references other shows, pop culture, and really anything there is to reference, this episode represents a true crossover because of Mulder and Scully's continuous presence in the show. Bonus crossover moment: it's really more of a reference, but Leonard Nimoy voices himself narrating the happenings in Springfield as an episode of a show similar to his actual 1970s series In Search Of....

Watch a clip of Mulder and Scully's animated investigation, complete with several characters from other sci-fi shows, here:

Mork & Mindy, Happy Days, and Laverne & Shirley
We all know Mork & Mindy was a spin-off of Happy Days -- Robin Williams's lovable alien Mork appeared on the episode "My Favorite Orkan" on February 28, 1978, when he threatened to force Richie back to his planet after Richie saw his spaceship. Mork then got his own show which premiered on September 24, 1978, and was set in 1978 as opposed to the 1950s, as Happy Days had been. The pilot episode of Mork & Mindy thus involved a crossover of epic proportions: towards the end of the episode, Mork explained to a doubtful Mindy that he has visited Earth before and introduced a flashback sequence in which Mork lands at the Cunningham's house in the 1950s and finds Fonzie (Henry Winkler) house-sitting -- they end up in a conversation about dating and Fonzie agrees to set Mork up. The date arrives -- and it's Laverne (Penny Marshall) from Laverne & Shirley, itself a spin-off of Happy Days! Hilarity ensued. This sequence was a surefire way to draw viewers to the newly-premiering show, and sure enough Mork & Mindy quickly topped the ratings charts. But what may have been conceived as a stunt landed a solid place in the annals of crossover history -- and our hearts.

Think we missed one? Tell us about your favorite crossover!