To say this is a powerful marketing tool is to completely understate it's value in driving dollar-spending traffic to your door!  Not only can you offer your customers special discounts, you can also drive them to your website and see everything else your business has to offer while they're there looking for their coupons.  This is also a great way to gauge the effectiveness of your other advetising.  For example, if you are telling people in your radio advertising to visit your website to print off special web-only coupons for good or services and you have thousands of people come into your store with coupons, I think it is safe to say that this advertising is working and the money you spent on radio is well worth the cost.  This is a great way to prompt users to act by offering them something special if they print out a coupon and come in the store to redeem it.  As already stated, this is a very powerful tool and can be even more powerful when used in conjunction with other advertising like television, radio and newspaper.