OTR capable clients
Pidgin open source IM client with OTR plugin.
Pidgin is an Open-Source IM client for Windows and GNU/Linux with a plugin for having OTR conversations over many protocols.
Adium - an open source client for Mac OSX.
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 - an OTR client for Jabber on Android and iOS.
ChatSecure, formerly Gibberbot, is a fully featured instant messaging application integrated with “Off the Record” capabilities.
Irssi-OTR - an Irssi OTR plugin.
Irssi-OTR is a plugin for the Irssi client which offers OTR functionality for those who prefer to work from their terminals.
WeeChat-OTR - an OTR script for the WeeChat client.
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.
Jitsi - Open Source Video Calls and Chat.
Secure video calls, conferencing, chat, desktop sharing, file transfer, support for your favorite OS, and IM network. All this, and more, in Jitsi.
Psi - Instant messaging as free and open as it should be
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.
CoyIM - A chat client that is secure and safe by default
CoyIM is a new chat client that is safe and secure by default: no settings to change, no plugins to install, no computer configuration to change. It uses OTR by default: no need to add a plugin.