TOC is an open protocol for AIM. It supports about as much as AOL's AIM 2.0 client does. For whatever reasons (probably security), America Oncrack didn't want to have an open specification for AIM, so they made a gateway to the one Windows and Mac OS clients use. TOC clients include gaim, TiK, and AOL's very own java client at http://toc.oscar.aol.com/. AOL ditched development of the TOC server, but it is still running and very functional. The protocol the official Windows and Mac OS AIM clients use is called OSCAR.

See also my writeup under TiK, because TOC and TiK's history are closely integrated (TiK/TOC was advertised as AOL's official UNIX port)

TOC is also a common abbreviation for table of contents. There is the "TOC header" on a CD or CD-ROM that tells track location and size information.