First I will admit that I had not one in my memory to report, but your question sent me to my bookshelves to look. After that I resorted to internet sleuthing. I found this and thought it too good not to share it with you:

The Lion, the Witch and the Wardrobe

To Lucy Barfield

My Dear Lucy, I wrote this story for you, but when I began it I had not realized that girls grow quicker than books. As a result you are already too old for fairy tales, and by the time it is printed and bound you will be older still. But some day you will be old enough to start reading fairy tales again. You can then take it down from some upper shelf, dust it, and tell me what you think of it. I shall probably be too deaf to hear, and too old to understand, a word you say, but I shall still be

your affectionate Godfather, C.S. Lewis

Long Day's Journey Into Night, by Eugene O'Neill:

        For Carlotta, on our 12th Wedding Anniversary

Dearest: I give you the original script of this play of old sorrow, written in tears and blood. A sadly inappropriate gift, it would seem, for a day celebrating happiness. But you will understand. I mean it as a tribute to your love and tenderness which gave me the faith in love that enabled me to face my dead at last and write this play -- write it with deep pity and understanding and forgiveness for all the four haunted Tyrones. These twelve years, Beloved One, have been a Journey into Light -- into love. You know my gratitude. And my love!

            Gene
      Tao House
     July 12, 1941

on the less serious side, I think the women who write paranormal romances have hilarious dedications. Dakota Cassidy for example "for my friends who love my craziness like a red-blooded male loves a Victoria Secret catalog... and always, Rob...he cheers my successes, soothes my fears, and knows nothin' says lovin' like a Starbucks white chocolate mocha."

Although it is not technically a dedication, Victor Hugo's preface to Les Miserables always sticks with me:

So long as there shall exist, by reason of law and custom, a social condemnation, which, in the face of civilisation, artificially creates hells on earth, and complicates a destiny that is divine, with human fatality; so long as the three problems of the age--the degradation of man by poverty, the ruin of woman by starvation, and the dwarfing of childhood by physical and spiritual night--are not yet solved; as long as, in certain regions, social asphyxia shall be possible; in other words, and from a yet more extended point of view, so long as ignorance and misery remain on earth, books like this cannot be useless.

Gino Strada's Green Parrots is a diary of his work as a surgeon in Iraq, Afghanistan et al., and most of the dedication is a love letter to his wife which is too long to repeat, but I've always liked the way it began:

"Usually, dedications are at the beginning of books.

Sometimes I have found them a bit annoying, sticky, as if they had not much to do after all with what was being dedicated: To mary with affection, and then there follows a treatise on the technology of the artificial reproduction livestock."