Occassionally restuarant.com offers 70% off on gift certificates. Since you normally pay $10 for a $25 gift certificate it's a nice deal.
Keep in mind that you usually need to spend twice the value of the certificate - so it's really more like a big coupon. I don't mind paying $3 for a $25 coupon.
The real treasures are when you find a $50 certificate for a fairly expensive restaurant.
You need to check the site occassionally as new deals are offered and the more desireable restaurants offer limited quantities.