The Pidgin chat client offers a way to connect to the OCS server through a plugin called SIPE.
Installing and setting up Pidgin for OCS access on Debian / Ubuntu based systems
First, make sure you have the latest versions of the following packages:
You can install them with the following command:
sudo apt-get install autotools-dev pkg-config libglib2.0-dev libgtk2.0-dev \ libpurple-dev libtool intltool comerr-dev
Next, you will want to install the latest version of Pidgin (at least version 2.7.x) and the Pidgin-SIPE plugin:
sudo apt-get install pidgin pidgin-sipe
After installing Pidgin and Pidgin Sipe, you will need to restart the computer.
Launch Pidgin (how to do this depends on the version of Ubuntu you are running, it should be located in Applications/Internet (Apps)/Pidgin Internet Messaging.)
When the Welcome to Pidgin screen appears click on the Add button.
For the Protocol section choose Office Communicator and fill out the form as so:
Finally, click on the Add button. This should add and enable your Lync account and connect you to the Lync Server.