tumnuslucy_fans: The ONLY LJ community for this pairing! Full of nice people, too. ;)
narnia_fiction: Sometimes has new Tumnus/Lucy stories up.
_jamesmcavoy: For fans of James McAvoy, who plays the very cute Tumnus!
georgiehenley: Georgie Henley community. (Georgie plays equally adorable young Lucy.)

General Narnia Fanfiction: For all the books and/or the movie. A few hundred Narnia stories to be found here.
C2: Tumnus/Lucy Love: Run by PuckHiggins, who has a story here and maintains the T/L LJ community.

A Walk in the Snow: A fanlisting for the Tumnus/Lucy pairing!
Chronicles of Narnia: Mr. Tumnus and Lucy: A Myspace group for them.
A Good Faun: The Mr. Tumnus fanlisting.
Simply Georgie: Fansite for this sweet little actress. (Lucy)
James McAvoy Online: Fansite for this Scottish actor with pretty blue eyes. (Tumnus)
NarniaWeb: Narnia movie news.
TheOneLion.net: Narnia movie news. Run by the same people behind TheOneRing.net!