Nalanda is a LMS written in PHP; supports common library functions such as issuing and returning books and user management, and also provides statistics.
Zim is a WYSIWYG text editor written in Gtk2-Perl; aims to bring the concept of a wiki to your desktop, and every page is saved as a text file with wiki markup.
Twinkle is a software phone for voice over IP communications using the SIP protocol; use it for direct IP phone to IP phone communication, or in a network using a SIP proxy.
Cabos is an open source P2P based on LimeWire and Acquisition which provides an easy-to-use UI, and iTunes + iPod integration; for Mac OS X, or Win2K users.