Are you looking for a 3rd party processor? Or your own merchant account? And... where are you located?
If you are a non-U.S. merchant, then PaySystems is likely a good option for you. 2CO is also popular but they have had a lot of DOS attacks lately and other issues.
If you are a Canadian merchant, you might want to check out PSI-Gate.
If you a U.S. merchant, under most circumstances you will be much better off with your own individual merchant account plus a gateway like Authorize.Net or some of the other options available.