I tried ADBLURB today and it worked for me - 20% off (the currency had to be US $)
These vouchers usually have a shelf-life, as do the sites they are on. However a Google search for "Blurb vouchers" will usually unearth something worthwhile - if you are willing to dig a bit.
I got 15% off today (14/03/12) with
I got 25% off (04/04/12) with