http://www.usaepay.com
http://www.plugnpay.com - I think their system is kinda messed up for membership sites.
http://www.eprocessingnetwork.com - The only problem with them is not really being able to do refunds through the gateway (you have to login to their admin area to do a refund).
If your looking for a system to take care of passwords and affiliates, you can go with
http://www.paywide.com
and they will set you up with what ever gateway you choose.