Connie Britton on Nashville
Death didn't keep Rayna Jaymes from a Nashville swan song when Connie Britton appeared in a flashback/dream sequence (and then for a cast-wide singalong).
Josh Dallas, Ginnifer Goodwin and more on Once Upon a Time
The final season of ABC's fairy tale hit was a reboot, but for the grand finale a large portion of the original cast returned to close the book on the series.
Josh Charles on The Good Wife
The character Will Gardner died in season five, but that didn't keep Josh Charles from showing up in the series finale. Dream sequences are powerful narrative devices!
Topher Grace on That '70s Show
After leaving for Africa at the end of season 7, Eric Foreman (Topher Grace) returned at the end of season 8 just in time to kiss Donna (Laura Prepon) and watch the arrival of 1980. Both Grace and Asthon Kutcher had left after season 7 to pursue other projects, though Kelso made appearances in several episodes in the final season, including the finale.
Ian Somerhalder, Elizabeth Mitchell, & Everyone on Lost
Almost all of the dead characters on Lost got to come back and be reunited with their soul mates in the final episode, including the ones played by Ian Somerhalder, Maggie Grace, Dominic Monaghan, Elizabeth Mitchell, Rebecca Mader, Jeremy Davies, and Cynthia Watros.
David Boreanaz on Buffy the Vampire Slayer
Despite being very busy on his own show, Angel (David Boreanaz) returned to Buffy the Vampire Slayer for the final two episodes of season seven. And it's a good thing too, because Buffy had to tell him she was cookie dough, and she was not done baking. He also had to hand off the amulet that would eventually allow Spike to save the world, but whatevs.
Josh Charles on The Good Wife
Two seasons after he was shockingly killed, Will Gardner (Josh Charles) returned as a ghost to guide Alicia Florrick (Julianna Margulies) through the show's series finale. Weird? Sure. Welcome? Completely.
Jennifer Morrison on House
Jennifer Morrison left House in season six (to, you know, save an entire universe of fairytale characters), and returned only once before the show ended in 2012 after eight seasons. She and Kal Penn both appeared in the series finale as visions that House (Hugh Laurie) was having as he tried to decide if his life was worth living.
Angus T. Jones in Two and a Half Men
Angus T. Jones did his best to leave Two and a Half Men totally behind around 2012, citing religious reasons, but did show up for a minute in the finale to reveal that Jake had left the army, moved to Japan, got married, inherited some step kids, and managed to gamble his $250,000 check from his supposedly dead father into $2.5 million. Not bad!
Rashida Jones and Rob Lowe in Parks and Recreation
Sure, Ann (Rashida Jones) and Chris (Rob Lowe) hadn't been gone all that long by the time Parks and Rec ended, but it was beyond great to see them again in the finale, and to learn that they were moving back to Pawnee...even if we wouldn't be around to watch them live there.
Everyone on Glee
Glee's series finale reunited the majority of the cast to find out how everyone was doing after college and beyond. We even got a glimpse of the unholy trinity—Quinn (Dianna Agron), Santana (Naya Rivera), and Brittany (Heather Morris) together again.
Sherry Stringfield, Eriq La Salle, Laura Innes, and Noah Wyle on ER
Aside from being Alexis Bledel's first post-Gilmore TV role, the 2009 ER finale was also a beautiful reunion for many of the show's original doctors, who all returned to attend a party to celebrate the opening of Dr. Carter's (Noah Wyle) clinic. Sherry Stringfield, Eriq La Salle, and Laura Innes all got back together with Wyle one last time, just missing George Clooney and Julianna Margulies' appearances just a couple of episodes before.
William Petersen and Marg Helgenberger on CSI
At least part of the old team got back together for the final episode when Catherine (Marg Helgenberger) was called to Las Vegas to deal with a bombing in her casino. Even Gil Grissom (William Petersen) found a way to get involved after leaving the show in 2009, and the series ended with Sara (Jorja Fox) quitting her brand new promotion and sailing off into the sunset with Grissom. Unfortunately, Nick (George Eads) was nowhere to be found.
Steve Carell on The Office
Michael Scott's (Steve Carell) surprise attendance at Dwight (Rainn Wilson) and Angela's (Angela Kinsey) wedding after being gone for two seasons was possibly the sweetest return of all, especially thanks to an incredibly well timed "That's what she said."