Good question, Princess_Luthien! In one of his letter, Tolkien says, 'I am a philologist, and all my work is philological'. A philologist is someone who studies old texts, like Beowulf and the Kalevala, and when Tolkien says his work is philological, I take that to mean that he is stating the fact that he uses ideas from other old writings to make his own writing. Some of these are:
Beowulf: Some of the names, such as Folcwald and Hama, come from this poem which Tolkien studied and wrote numerous essays on. There was also a scene in Beowulf when a slave steals a cup from a dragon hoard, like Bilbo and Smaug.
Kalevala: Kullervo, as Eryan said. Also there is an old man in the Kalevala who uses magic and is reminiscent of Gandalf. And there is a magical object which gives its owner great wealth but is destroyed in the end to ensure peace...and Norse mythology influenced him as well.nn[Edited on 2/11/2002 by Samwisegamgee]