Earrings that permanently fuse beings together
Potara Earrings are artifacts of the Kaioshin (Supreme Kais). These earrings are worn by all Kaioshin, as part of their standard outfits. When one pair of Potara Earrings are worn by a single person they have no special properties. However, if two individuals each wear one earring from a single Potara set on opposite ears, the true power of the artifacts is revealed. The earrings have the ability to fuse two people into a single entity, substantially enhancing the abilities of both individuals. The resulting fusion lasts forever, even if the fused being removes the earrings.
Potara Fusion is similar to the Fusion Dance technique, however unlike the Fusion Dance, the Potara Fusion is permanent. An exception to this was discovered when the fused character Vegito allowed himself to be absorbed by Super Buu, and was separated into Goku and Vegeta, the two individuals who had fused to form Vegetto.
There are three known Potara Fusions seen in the series.
Chronologically, the first was a fusion between Elder Kai and an old witch.
The second was the accidental fusion of Supreme Kai with Kibito, to form Kibitokai (NOTE: The fusion itself was not an accident, but they did not know the fusion was permanent, and merely wanted to try it out).
The third Potara Fusion was the most prominent. Goku fused with Vegeta to form Vegito.
One note of interest about the Potora Earrings is that unlike the Fusion Dance, the more dominant spirit of the two fused individuals takes precedence over the weaker not just in looks but in voice as well. The proof of this is that both Elder Kai and Supreme Kai seem to be the ones in control of their fused forms while having a few noticable traits from the old witch and Kibito respectively. One could speculate that Goku and Vegeta were matched equally when they fused, which may explain their doubled voice and flawless unison of bodies and clothing.