I'm 5'7 and pretty nimble, so I don't have a problem on most horses if I have stirrups - I'm no good climbing up bareback though. I've used the fence, and a salt block to climb onto the backs of horses..
At my barn we have a two step wooden one that the barn owner made a couple of weeks ago. Before that we had a big plastic one, but I'm assuming it belonged to another boarder who took it with her when he left. Before that we had another wooden one, but the BO ran over it with the tractor so he honored our request for a new one
I'll use anything as a mounting block- a fence, car, bucket, pile of dirt, etc. I can get on most horses easily from the ground, but I'd prefer to avoid the strain on the horse's back if I don't have to. I do agree that any horse should be trained to remain nice and calm whether mounting from a block, some random object, or the ground, as well as from both sides. You never know when you'll end up in a strange situation!