Pidgin is a chat program which lets you log into accounts on multiple chat networks simultaneously. This means that you can be chatting with friends on XMPP and sitting in an IRC channel at the same time. Pidgin runs on Windows, Linux, and other UNIX-like operating systems.
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Somewhat different. Matterbridge makes a connection between one or more chat rooms, while Spectrum2 allows you to connect as a single user to a different network.