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TCLink Credit Card Processing API
TCLink is an LGPL'd thin client API which can run credit card transactions over TCP/IP. A similar package is Red Hat's ccvs, however, it requires that you use a modem or leased line. With TCLink you can run credit card transactions directly over the Internet.
URL: http://www.trustcommerce.com/tclink.html
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Cost: Free (LGPL Platforms: Unix Version: 3.3