- Old Tom Bombadil is a merry fellow;
- Bright blue his jacket is, and his boots are yellow. — Tom Bombadil, The Fellowship of the Ring
Some information has lead many people to think that Tom may in fact be an incarnation of Eru Ilúvatar, the creator of Arda, including his ability to hold the One Ring of Power and think of it as just a pretty shiny that he would probably lose down the back of the couch. There's also his Elven name, Iarwain Ben-adar, which loosely translates to "oldest and fatherless." Tom Bombadil is his Bucklandish name; to the Dwarves, he is Forn; to the Northern Men, Orald. He is the eldest being in Middle-earth, that's a fact. Whether he is an incarnation of Eru or just a Maia is a mystery.
For more information, see his page on the Tolkien Gateway.
He inexplicably makes a cameo in My Immortal, though this is likely to simply be the author mis-typing the name of Tom Riddle.