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.
Secure video calls, conferencing, chat, desktop sharing, file transfer, support for your favorite OS, and IM network. All this, and more, in Jitsi.
Psi is a free instant messaging application designed for the XMPP network. Fast and lightweight, Psi is fully open-source and compatible with Windows, Linux, and macOS. It has a plugin for OTR.