Here's several sources of commercial and free ATM software on the net.
Commercial software is labeled as such.
The following text provided by Chuck Cranor . This site also includes references to the ATM on Linux mailing list, archives for this mailing list, and several relevant research papers. The toolkit provides an environment for experimentation with ATM adaptation layers, interoperable ATM signalling, IP over ATM, and real time transport protocols. Data Connection is a commercial supplier of a wide range of portable communications software, including ATM, SS7, SNA, Messaging, Directory and Data Conferencing products. IDLInet Native mode ATM protocol stack.