Totally depends on the horse - the barn manager at my barn always goes and gets coffee before he comes in and he always picks up half a dozen donuts. If you come in early and help him, you get a donut. One of the donuts always goes to one mare in particular, though. Every morning she whinnies when he comes in, he says good morning to her and gives her her donut.
One of the geldings at my barn is crazy for bananas and the one I lease would do anything for mint. They tend to like sweet things like apples, bananas, etc. They did a study that said that horses prefer garlic flavoured foods, and the results were as followed: fenugreek, banana, cherry, rosemary, cumin, carrot, peppermint and oregano. Apples didn't even make the top 8!
Most treats are okay, just watch out for sugar infused commercial things like Likits: the sugar won't really hurt them but it's suicidal for their teeth.
Oh, and most things that are poisonous to horses are things you would never think of feeding them and are mostly flowers and plants found in the wild, stuff like: wort, oak, foxglove, poppies, hemp, nightshade (all types) among others. You should try to avoid tomatoes, as they are a member of the nightshade family. Potatoes are also not recommended along with flax. Basically, try to avoid feeding your horse flowers and greenery (and this is why it's SO important that your horse not eat on trail rides) and you should be fine.