Avatar: The Last Airbender/Shout-Out

In the first season finale, Pakku tells one of his students that in a few years he might be able to defeat a sea sponge. The creators later admitted in a commentary that it was a reference/friendly poke towards the fellow Nicktoon, one of its main competitors. The second season also contained a shout-out to its cancelled, fellow Periphery Demographic Nicktoon Invader Zim, for whom Avatar co-creator Bryan Konietzko had been an art director, as well as numerous shout-outs to its fandom, especially to the cosplayers and fangirls.
"Nightmares and Daydreams" has Aang imagining Momo as a samurai, who looks very similar to Miyamoto Usagi.
When Toph tells Aang, "You da man, Twinkletoes!", she flashes the Buddy Christ sign at him. It's a little difficult to make out, since she's aiming right at the camera and not slightly to the side, but it's there; she's pointing with the right hand and holding a thumbs-up close to her body with her left.
The Sandbenders, bandage-wrapped desert nomads, look like the Tusken Raiders of Tattooine. Possibly a double shoutout, because not only did the Sandbenders' appearance call to mind the Tusken Raiders (sand people, sandbenders... huh.), but the way they were shown in their first named appearance (where the archaeologist shoos them away from Appa outside the ice spring fruit bar) is very similar to the treatment of Jawas.
Toph calls Ba Sing Se the Worst. City. EVER!
In the finale, Toph calls Sokka "Captain Boomerang".
Also from the finale, Phoenix King Ozai says, "It's time for this world to end in fire," sounding very similar to part of a the Robert Frost poem "Fire and Ice".
The "Love and Peace" line in the finale is a Shout-Out to Vash the Stampede, who used the line "Love and Peace" frequently in Trigun. All Zuko lacked was the "Peace Sign" hand gesture.
All of the shoutouts and affectionate parodies of Professional Wrestling in the Earth Rumble crew.
The creators stated that they were also fans of Cowboy Bebop, and decided to have the forest where Aang and Ozai fight named after the series currency, Wulongs. Then there's Sokka's Cactus Tango.
Wan Shi Tong, the Knowledge Spirit who looks like a giant barn owl. When he walks with his back turned, he looks almost exactly like the character No-Face in the Hayao Miyazaki movie "Spirited Away".
The shout out to Miyazaki's works happens again when Aang successfully defends the Northern Water Tribe from a Fire Nation siege by having the Ocean Spirit merges with him, becoming a giant, glowing aqueous creature that is an obvious shoutout to the Forest Spirit's night form^{[dead link]} in "Princess Mononoke". The concept of a human trying to kill a nature spirit also comes from the movie.
The scene in Wan Shi Tong he stalks Katara through the library is very reminiscent of a similar scene in a well known movie that also featured "raptors".
Katara parting the waters in "The Serpent's Pass" doubles as a shout out to both The Bible and The Prince of Egypt. The latter only because of the serpent's silhouette showing the waters in the same manner that a shark's did in the movie.
The gate to the Serpent's Pass tells people to "abandon hope" as do the gates of Hell in Dante's Inferno.
Iroh tells Zuko he needs to ask himself the "big questions": "Who are you?" and "What do you want?"
When the unagi rises out of the water and is seen in profile, it's quite reminiscent of Unit 01.
When Katara, disguised as the Painted Lady, begins approaching Fire Nation soldiers a child says "She's com-ing!" in the exact same tone as the line |"They're he-re!"
Toph calls Aang "Twinkle-toes" during Earthbending training. R. Lee Ermey used this as an insult against the new recruits in Full Metal Jacket, and Toph's use of "Twinkle-toes" fits her Drill Sergeant Nasty attitude.
Right after Aang saves Sokka and the fisherman in "The Storm", a massive wave sweeps up their boat in a manner similar to The Perfect Storm.