There are numerous gateways in the United States - see what your merchant account provider supports. Some others are Quantum, LinkPoint / YourPay / First Data Global Gateway, Payflow, TrustCommerce, Yahoo! etc.
Here is a list I had made a couple of years ago. Some have merged / been bought out, but it hopefully gives you an idea of what is out there
[list][]ALOHA - Nashville, 4000
[]ATOMIC SOFTWARE - Nashville, 4000
[]ATS/GLBL COMMERCE 2000 - Nashville, 4000
[]BLUE MONEY - Nashville, 4000
[]CARDSERVICE P.C. - Omaha, 4010
[]CREDIT LINE - Nashville, 4000
[]DATACAP - Nashville, 4000
[]Digital Dining - Nashville, 4000
[]DPOS - Omaha, 4010
[]I.C. VERIFY - Omaha, 4010
[]IBILL - Nashville, 4000
[]IC Verify Purchase Card Level II & III - CardNet, 206
[]IC Verify/VAR - CardNet, 206
[]IC-VERIFY DOS - Nashville, 4000
[]IC-VERIFY WINDOWS - Nashville, 4000
[]Image Tech / Visual Matrix - Nashville, 4000
[]IPN/Aloha/VAR - CardNet, 206
[]IPN/PC Charge/VAR - CardNet, 206
[]LP Virtual Terminal - Nashville, 4000
[]MAC AUTHORIZE - Omaha, 4010
[]MERCHANT MASTER - Omaha, 4010
[]MICROS 2700 / 3700 / 4700 / 8700 - Nashville, 4000
[]PAYLINX - Nashville, 4000
[]PC AUTHORIZE - Omaha, 4010
[]PC CHARGE - Omaha, 4010
[]PC-AUTHORIZE WINDOWS - Nashville, 4000
[]PC-CHARGE / NASHVILLE - Nashville, 4000
[]PTC - Omaha, 4010
[]SABLE - CardNet, 206
[]SHIFT 4 $$$ IN THE BANK - Nashville, 4000
[]Southern Datacom / Protobase - Nashville, 4000
[]SQUIRREL - NATIVE - Nashville, 4000
[]SQUIRREL - VISA - Nashville, 4000
[]TRANSACTION PLUS - Nashville, 4000
[]USA E PAY - Nashville, 4000
[]VAR - CardNet, 206
[]ANACOM - Nashville, 4000
[]API/Virtual/Basic - Nashville, 4000
[]AT & T - Nashville, 4000
[]AUTHORIZE NET - Nashville, 4000
[]Bookkeeper / Lkpt Gtwy - Nashville, 4000
[]CC HOSTING ENGINE - Nashville, 4000
[]CYBERSOURCE - Nashville, 4000
[]DIRECT LINK - Nashville, 4000
[]HTML & VIRTUAL LINKPOINT - Nashville, 4000
[]IC_VERIFY INTERNET DIAL - Nashville, 4000
[]INTELLIPAY - Nashville, 4000
[]Jettis - Omaha, 4010
[]LINKPOINT API & VIRTUAL LINKPOINT - Nashville, 4000
[]LinkPoint Cart - Nashville, 4000
[]LINKPOINT GTWY (SWIPED) - Nashville, 4000
[]LPAP / VLP / SCRUB - Nashville, 4000
[]LYCOS-1 - Nashville, 4000
[]LYCOS-2 - Nashville, 4000
[]NET BILLING - Nashville, 4000
[]NETSCAPE - Nashville, 4000
[]ORBIT PAYLINK - Nashville, 4000
[]PAYCOM - Nashville, 4000
[]PAYMENT NET - Nashville, 4000
[]PC-CHARGE INTERET DIAL - Nashville, 4000
[]PURE PAYMENT / IMALL - Nashville, 4000
[]RODOPI GATEWAY - Nashville, 4000
[]RODOPI PAYMENT GATEWAY - Nashville, 4000
[]TELECHARGE IVR - Nashville, 4000
[]VERISIGN LINK - Nashville, 4000
[]VERISIGN PRO - Nashville, 4000
[]VIRTUAL CHECK - N/A
[]VIRTUAL LINKPOINT - Nashville, 4000
[]YAHOO - STORE - Nashville, 4000
[]YAHOO GATEWAY - Nashville, 4000
[/list]
There are probably more now and I think I just grabbed that from one platform (transaction processor). I probably need to update it but it is a start. You can also look on some of the shopping cart’s features and get an idea of the gateways there