Users can fuse together objects, powers, emotions, concepts, living beings, or even themselves with something else, turning into single unit. They can also combine their physical attributes, abilities, and powers together to form a new being that is stronger, faster, smarter, and more powerful than they were alone.
Locacaca Fruits (JoJo's Bizarre Adventure Part VIII: JoJolion)
Wall Eyes (JoJo's Bizarre Adventure Part VIII: JoJolion)
Cubit (Lego's Mixels)
Talisman of Kamar-Taj (Marvel Comics)
Time Teleporter (Monster High: Freaky Fusion); through accident
Combine-Inator (Phineas and Ferb)
Ruber's potion (Quest for Camelot)
Transporters (Star Trek); through accident
Millennium Scale (Yu-Gi-Oh!)
Polymerization card (Yu-Gi-Oh!)
Orb Ring (Ultraman Orb)
Firestorm Matrix (Arrowverse/DC Comics)
Ben 10,000's Biomnitrix (Ben 10: Omniverse) can fuse any two Omnitrix aliens into one transformation.
Ben 10 (Heroes United) turns into the alien Upgrade and fuses with Generator Rex to create the Upgrade Suit.
In order to free himself from all his grief and emotions caused by him losing both his family and friends, Danny Fenton had Vlad split his human half from his ghost half, but freed from his emotions, his ghost half merged with Vlad's ghost half…
…Danny's ghost half filled with all his negative emotions was further corrupted through him fusing with Vlad's evil ghost half, resulting in the creation of the most powerful and sinister ghost to ever to exist on the planet…
…wreaking havoc across both the Human World and Ghost Zone for ten years, Dark Danny became the evilest and strongest ghost in existence.
The Crystal Gems (Steven Universe) are members of a race of magical aliens capable of fusing together with one another, creating different variants of fusion-beings.
Steven and Amethyst (Steven Universe) fuse to form Smoky Quartz.
Boxman's robots (OK K.O.! Let's Be Heroes) combined into Boxbot Prime.
Ruber (Quest for Camelot) uses a special potion to merge human beings with weapons...
...doing so with himself and Excalibur to ensure no one would ever take it from him.
Brainiac and Lex Luthor (Justice League Unlimited) fuse to become one being known as Luthoriac.
Feist (Archie's Sonic the Hedgehog) was able to condense all of the thousands of Chaos Emeralds across the universe into seven differently colored gems.
B'Wana Beast (Batman:Brave and the Bold) can fuse any form of living creature, including humans, together into new creatures, such as this "arequinid", a combination of a horse and a spider.
The Firestorm Matrix (DC Comics) enables Firestorm to merge with other beings.
After absorbing energy from other kaiju, an Alpha Trilopod (Godzilla: Rulers of Earth) merges with a Beta Trilopod...
...creating a new Trilopod with both the powers and physical characteristics of the target kaiju.
The Deathwalkers (Marvel Comics) used the Talisman of Kamar-Taj to merge into...
...Deathwalker Prime, a powerful monstrosity with total control over the four classical elements, and thus mastery of the fifth: the soul.
Scarlet Warlock (Marvel Comics) was an alternate version of the Scarlet Witch who attempted to transfer Wolverine's Adamantium skeleton into Magneto's body to make her even more powerful...
...but failed, resulting in the accidental fusion of Wolverine, Magneto, Quicksilver, Mesmero, and Scarlet Warlock into a single entity known as Brother Mutant.
Coyote Starrk and Lilynette Gingerbuck (Bleach) used to be a single being that split itself into two...
...to use their full powers they must merge.
Tatsumi Oga and Baby Beel (Beelzebub) have merged together through Super Milk Time.
A Xros Loader (Digimon Xros Wars) has the power to fuse any Digimon.
Omnimon (Digimon) the fusion of WarGreymon and MetalGarurumon.
Rafflesimon (Digimon) the fusion of Lotosmon and Rosemon.
Mastemon (Digimon) the fusion of the angelic Angewomon and demonic LadyDevimon.
GraceNovamon (Digimon) is the fusion of the god of the sun Apollomon, and the goddess of the moon Dianamon.
Omnimon Merciful Mode (Digimon) is the fusion of Omnimon, Seraphimon, Rosemon, Vikemon, HerculesKabuterimon, Hououmon, and Magnadramon.
Ordinemon (Digimon) is the result of a forced, dark fusion between Raquelmon and Ophanimon Falldown Mode.
Nail (Dragon Ball Z) uses Namekian Fusion to merge with Piccolo...
...and later, Piccolo with Kami.
Trunks and Son Goten (Dragon Ball Z) performing the Fusion Dance to form Gotenks.
Potara Earrings (Dragon Ball Z) can fuse any two individuals together when said individuals each wear one earring on opposite ears...
...Vegito is the result of Son Goku and Vegeta wearing the Potara earrings...
Merged Zamasu (Dragon Ball Super) is the result of Goku Black and Future Zamasu using the Potara earrings.
Kefla (Dragon Ball Super) is the Potara fusion of Kale and Caulifla.
Gogeta (Dragon Ball Super) is an alternate fusion between Son Goku and Vegeta by performing the Fusion Dance.
Shu Ouma (Guilty Crown) is able to fuse two or more Voids into one; here, he fuses the Voids of Kenji Kido and Inori Yuzuriha into a massive energy cannon.
Locacaca Fruits (JoJo's Bizarre Adventure Part VIII: Jojolion) that fused Josefumi Kujo & Yoshikage Kira into Josuke Higashikata.
At the end of the anime, Snow and Koyuki (Marchen Awakens Romance) become one being so they can live with Ginta in his world.
Hiruko (Naruto: Shippuden) acquired a technique that allows for the fusion between different creatures.
Kisame Hoshigaki (Naruto) fused with his blade Samehada.
Madara Uchiha (Naruto) equips his Susanoo armor onto Kurama/Nine-Tails to increase offense, defense and control.
Sasuke Uchiha (Naruto) equips his Susanoo armor onto Kurama to increase offense, defense and teamwork.
Using the Burst technique, Ryouga (Pokémon RéBURST) fuses with Zekrom.
Gyokuro Shuzen (Rosario + Vampire) merged with Alucard.
Lady Oyakata (Rosario + Vampire) fused with her plants.
Polymerization (Yu-Gi-Oh) takes two Monsters and fuses them into one Fusion Monster.
Appli Drive (Digimon Universe: Appli Monsters)
The Pirabbid Plant (Mario + Rabbids Kingdom Battle) is the result of Spawny fusing a Rabbid with a Piranha Plant.
Ultron Sigma (Marvel vs. Capcom: Infinite) is the result of Ultron and Sigma fusing.
Lex Luthor and Brainiac finally become one - Justice League Unlimited
SCP-049 illustrated (The Plague Doctor) Updated ft. TheVolgun, Site-42, Dr Cimmerian & Dr Viewless