yaxim - Yet Another XMPP Instant Messenger
yaxim (Yet Another XMPP Instant Messenger) is a Jabber/XMPP client with open source (GPLv2). Why pay for SMS if you can have unlimited messaging on your data plan?
yaxim aims at security, low overhead and keeping your server connection open. So far, it only supports a single account.
Yaxim is Open-Source. You can contribute too!
I like Yaxim a lot, and would like to see multiple accounts support in it. The interface is clean, notifications are nicely visible and configurable. The default fonts seem a bit too big which should also be configurable.
Setup is very simple and quick, yet further configuration options are possible once an account is created.