Peter Jackson
Peter Jackson, the director of this franchise, is in every one of these films. He is always in the center of the shot and very noticeable to the audience, much like Hitchcock was in any of his cameos. In The Fellowship of the Ring, Jackson was a man in the town of Bree. He is un-credited, but his character's name was Albert Dreary. In The Two Towers, Jackson plays a Rohirrim warrior who throws a spear at the Uruk-hai that charge Helm's deep. Finally in the third installment, Return of the King, Peter Jackson is a mercenary on the black ships who is killed by Legolas's arrow. Look for him in the Hobbit films.
Alan Lee and John Howe
Alan Lee and John Howe are the two main concept artists for all of the Tolkien films. Their artwork inspires all the sets that are built for the films. More the artist type, I was surprised to see them taking cameo roles as well. Both play men of the chosen nine to bare rings in Fellowship and Alan Lee also plays a solider for Rohan in the battle of Helm's deep.
Royd Tolkien
Royd Tolkien is the great grandson of author J. R.R. Tolkien. That's right, the author of this little series. Royd was given the opportunity to play a solider of Gondor in Return of the King.
Billy and Katie Jackson
Billy and Katie are Peter Jackson's two children. You can find them in all kinds of scenes throughout the trilogy, and I personally find it awesome that they got to participate in this franchise.
Hannah Wood
Hannah Wood is Elijah Wood's little sister. She also gets to be an extra in the Helm's deep scene. Yay for family projects!
You never know what you may find when you rewatch a film you've seen a hundred times before. Keep your eyes open and remember to Watch Learn and Love.