Sorry, this is late notice if your birthday happens to be January 1. Disneyland is offering free admission on your birthday in 2009!
You have to register on the Disney website here. In other words, you can't just show up on your birthday and expect to get in free--unless you've already registered. You also have to show picture ID at the gate.
For more information, and to register for your free birthday ticket, click on the link above.
And Happy New Year! Best wishes for an affordable 2009!