Remobo

Send and receive files on your own VPN

Messenger Plus!

Plugin for Windows Live Messenger with more skins and custom text
VIPole Secure Messenger

VIPole strongly encrypts messages, group chats, voice/video calls and files from prying eyes
WhatsApp

A messenger app that started life on mobile devices but is now available for PCs
Yahoo! Messenger

Full version of this popular chat client that includes the latest features