Canon offers its refurbished Canon EF 17-40 f/4L USM Lens for $589.99. Coupon code "PAYPAL10" cuts it to $530.99. With free shipping, that's $9 under our February refurb mention and the lowest price we've seen for this lens in any condition. (It's also $89 below the best price we could find for a new one today.) This ultra-wide-angle lens fits all Canon EF mount SLRs and has a 77mm filter.
Note: A 1-year Canon warranty applies.