Maybe the most well-known Amulet, the Alicorn Amulet. It was accused for corrupting Trixie, but some would claim it was her impure purpose and feelings that caused her all the problems.
An amulet strengthen and improves magical powers, improving on the performance, channelling magical power for the wearer, or user, but does so, only based on what nature of the Amulet was originally fashioned into.
In Trixie's case, her feelings were foul, hate and jealousy ruling her actions out of proportion. Pushing the rest of her out of the way, focusing on the motivation behind the act easily pushed her well out of character, which later caused her to back-step, once she was freed of the Amulets influences, and her focus was reset to her normal self.
There is the Amulets of the Elements of Harmony, supposedly ruled by the Element of Magic. Known to have been wielded by Celestia and Luna in the past, until the fateful day when Celestia had felt herself pushed in the corner and thus forced into use all the six of the Elements in order to banish Luna to the Moon.
Since harmony draws its power from harmonious cooperation, it would have to be at its best, when the group of wielder work together, and the better they did so, in a tight-knit friendship, the stronger the effects would be.
This particular artefact was made to handle six elements, and thus it would be bound to be at its strongest with six ponies wielding it.
There is the Amulet Zecora asked Twilight Sparkle to use against Trixie later, that is suspected to have no real power, but this far that has never really been proven.
These Artefacts relay on a wearer, or user to amplify powers, but apparently very little had been known among the general populous.
If an Amulet is focusing a purpose, or drawing from feelings, they can be very powerful, and ultimately dangerous to wield, since excess is easily coming from these users.
If it is built upon the Harmony, it may be equally dangerous to rely upon, since missing one Element, or not being fully in tune, and you may lose everything, thus putting false hope to what you never really had.
Certain special materials are known to enhance the effects of an Amulet of Tiara, when they work magic.
If an Amulet is based around a state, like Naivety, it would easily find shortcuts, making spells simpler to use, then a Unicorn normally, or naturally would pick up on, though if done wrong, this could only be used for a few years. If coupled with a well-balanced personality with joys and a certain level of maturity and good judgement, it may still work well beyond expected ranges.
Elemental Artefacts could lend the wearer the aspects of being a different type of pony, so that an Earth Type Pony could be allowed to live in the clouds and fly as if born a Pegasus, though these commonly would work better for some, and most likely if the wearer had Pegasi ancestry. Though that may be speculative at best.
A Harmony Amulet would never truly hurt, since hurt doesn't only be against the purpose, it would hurt the harmony it was set up to create and strengthen, or even destroy it entirely.
Speculations has it, there may be numerous other types. Effects and purposes could be mixed almost endlessly, with the skilled crafter as the only true limitation, though materials used could damage, or strengthen the effects to a certain degree.