Originally Posted by Konda

I think http://www.cybersource.com is the best one around.
Before you send a transaction to the bank, you can send it trough their system, besides a negative cc database, they also check lots of other factors. They will return a 'score' and you can decide what the minimal score should be to decline it right away or forward the transaction to the bank.

I think ccbill might use them as well although I am not sure.