google messages

Send messages and make calls for free to other Viber users.

Usenet newsreader handles messages, files, & media.

Write secret messages to your friends.

Embed hidden messages inside regular emails (designed for kids).

Encrypt instant messages with Open-Source security.

Free software to send free SMS (and MMS) messages from your Mac

Add cards in Address Book from selected Mail messages.

A plugin for Apple Mail that shows messages

It lets you send/receive messages instantly and participate in chats.

Quickly send messages to your Adium contacts.