From now through April 16, 2012, Office Depot will shred up to 5 pounds of your documents. Just bring them in to your local Office Depot's copy services center. I checked Office Depot's website but didn't find the promo. However, you can read details in this article at the Miami Herald newspaper's website HERE.
Office Depot's site does say that their everyday price for shredding is $0.99 per pound (at this Office Depot link). While there, you can also copy your 2011 tax return for free (up to 25 single-sided black & white copies permitted for FREE).