Humans smile, cats smile. I guess other primates smile, but are there any other mammals that actually smile?
Some dogs sure appear to when they’re playing.
The ‘smile’ observed by scientists in chimps and some other non-human primates is an expression mean thought to mean ‘I won’t hurt you, please don’t me!’ it is normally made only to others of it’s own specis and only by the smaller or less powerful party. This is thought to be the origin of the human smile (ie going from I won’t hurt you to I am friend).
I have a dog (a lab) who is very playful and often seems to smile but I am inclined to put this down to wishful thinking.
Dolphins appear to smile. This is simply due to the shape of their mouths, not an emotional state.
Some animals bare their teeth in a communicative way, but not as a smile.
This is what I was thinking about dogs and cats.
With cats you can tell when they’re unhappy, but when they’re happy, they do tend to smile - perhaps they’re learning from us.
Have you ever seen a dead dolphin? It’s really odd. They still have that cheeky grin, even when thair dead (unlike cats and dogs who go flopy in the face).
Not exactly a double blind study, but I had a dog that definatly smiled.
Experiment went like this:
Me and buddy in bar.
Me; Bullshit, dogs can smile
Friend; no you are misinterpreting some sort of behavioral action your dog is exibiting, no emotional reaction is present
Me; lets take a ride to my place
Ride takes place, dog runs to gate
Friend; Holy shit, that dog is smiling!
Granted this was a special dog. Her mother died at birth and I bottle fed her. But she still was a dog and there is no doubt in my mind that the lip stretching, tail wagging behavior wasn’t an emotional response.
Pretty much the same gesture I give someone I am glad to see. (well, minus the ass wiggle)
Only humans can smile. It is however such an important non-verbal message, that we tend to recognize it even when there is no smile. Best example of our powerful imaginagion are smileys: 
Now is there actually an emotion shown in these smiley pics? No, it’s all in our head, yet so easily to be recognized, that we have found another name for smileys: emoticons. This is a better name for them, as they are indeed iconic illustrations of (human) emotions (communicated through facial expressions).
Not even chimps, that are so close (evolutionary) relatives to us, do actually smile. The facial expression that comes closest to our smile is actually a “showing teeth” gesture, which means nothing friendly, but instead “Beware, I’ll defend myself!” But we humans have evolved, and we no longer “speak chimpanese”, therefore we misread it as laughing or smiling, while it means just the opposite.
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