A common device in tales of horror is to make the mundane and ordinary creepy. The more innocent the thing or character, the more shocking its transmutation to evil. Many horror movies or films features evil children (Damien in The Omen), dolls (Chucky in Child's Play), puppets (Puppet Master) and other toys. Clowns and Santa Claus (in Silent Night, Deadly Night) also feature as childhood icons turned evil.
As at the end of Season Two of Supernatural, there have not been any "evil" toys. However the meaning they have been imbued with through their status as a horror movie trope means that their appearance in an episode immediately makes the viewer anticipate that something terrible is about to happen. This is seen in the focus on nursery toys in flashbacks to Sam's nursery in the 1.01 Pilot, and in Rose's nursery in 1.21 Salvation.
The cymbal playing monkey appears in 1.09 Home and is a reference to a short story by Stephen King titled "The Monkey", synopsis as follows: "The story centers on a toy monkey that is apparently possessed by evil. The monkey is found by children, and their parents are, for some reason, freaked out. Periodically, the sinister monkey comes into motion, predicting the death of someone. In the end, the monkey is dumped in a lake in a bag full of heavy stones, thus stopping the deaths."
In 1.09 Home, the monkey starts playing just before the poltergeist causes the plumber's hand to be mangled in the garbage disposal.
Jensen and Jared both report finding the dolls in 2.11 Playthings scary.
Jared: Dolls. That was spooky. Just walking into a room full of dolls.
Jensen: Walking in with a bunch of dead eyes looking at you.
Jared: Every now and again, one of them was missing one eye, or there was one that didn't have either eye.
Jensen: Just two empty sockets.
Jared: And Jensen was like, "Put your fingers in her eyes." And I was like, "I don't want to. What if something gets me?" For whatever reason, we couldn't force ourselves to.
Jensen: Because you're a pansy.
Jared reports sneaking a doll into Jensen's trailer to freak him out.
The teddy bear come to life in 4.08 Wishful Thinking also plays on the concept, by making a child's toy the embodiment of existential angst.
A doll's head is used to scary effect in 4.11 Family Remains.