Pidgin is an Open-Source IM client for Windows and GNU/Linux with a plugin for having OTR conversations over many protocols.
Adium is a free and open source instant messaging client for OS X. It has support for various IM protocols and comes with OTR by default.
ChatSecure, formerly Gibberbot, is a fully featured instant messaging application integrated with “Off the Record” capabilities.
Irssi-OTR is a plugin for the Irssi client which offers OTR functionality for those who prefer to work from their terminals.
WeeChat is a commandline interface client. It runs on many platforms like Linux, Unix, BSD, GNU Hurd, Mac OS X and Windows (cygwin). Their script ecosystem allows for using various protocols like Jabber and IRC. Also OTR.