Suggestion

Suggestion is a rare ability allowing users to project themselves as an inner voice to others, implanting incredibly strong suggestions into their minds. Victims will hear the user's suggestions as their own, believing that the thoughts and ideas they have are their own.

This power can be used to plant thoughts and ideas, but does not give the user control over the minds of others, as pointed out by Leo and demonstrated by Cupids.

Guardian Angels
"Well, guardians did more than keep you upright. They're your inner voice, your conscience, instincts. Without them you tend to get a little lost."

- Kyle to Paige.

Guardian Angels are protective beings who protect people in their every day life. They use this to whisper warnings or advice to their charges when they are in danger.

For instance, if a man is in danger of being hit by a car because he's about to cross the road without looking, his Guardian Angel will appears before him and suggests that he stop and look before crossing the road.

Spirit Killer (Darklighter)
The Spirit Killer was a darklighter who could project as an inner voice to their victims, speaking words of discouragement and the like into their ears, making it seem as if their cursed actions came from their own ideas and intentions, which eventually lead them to the point of suicide.

Barbas
"So the fear that you're not good enough, that you're not worthy of the power of three. Oh, go on, prove yourself. Save Cole the same way Phoebe did before. Your sisters will be so proud."

- Barbas using his power on Paige.

Barbas was able to utilize this ability after he learned how to astral project and remain invisible. Together with his power to read fear, he was able to project as an inner voice to his victims, preying their fears and doubts, and then suggest ways—which would usually end up destroying them — to overcome it.

Cupids
"This is just a normal attack of cold feet. Just remember all the things you've experienced together and let those feelings guide you."

- Coop using his power on Mitchell to help him get over his cold feet.

Cupids and other beings in possession of a Cupid Ring can use it to slip into people's hearts and minds to implant thoughts, influencing their moods, behaviours and actions. The ring is mainly used to implant feelings of love into a person mind, but in the wrong hands, can be used to suggest reasons to hate as well as to harm others.

A common practice among cupids is to first slow down time when making love connections, and then project as an inner voice to charges, setting them on the path to love. Leaving their charges unaware of their presence and influence. However, Coop used suggestion twice without slowing down time, first on Mitchell and then on Bree.

Rex Buckland
Rex Buckland had this power. Using it along with his telepathic abilities and Astral Projection, he was able to manipulate Prue into stealing a valuable tiara from the auction house, make Phoebe be a witness to what was seemingly the proof Prue killing someone, and nearly allowed Andy and Darryl to find the tiara in the manor.