DirSync Pro (Directory Synchronize Pro) is a small, but powerful utility for file and folder synchronization. DirSync Pro can be used to synchronize the content of one or many folders recursively.
MediaTomb is an open source (GPL) UPnP MediaServer with a nice web user interface, it allows you to stream your digital media through your home network and listen to/watch it on a variety of UPnP compatible devices.
Enlightenment is not just a window manager for Linux/X11 and others, but also a whole suite of libraries to help you create beautiful user interfaces with much less work than doing it the old fashioned way and fighting with traditional toolkits, not to mention a traditional window manager. It covers uses from small mobile devices like phones all the way to powerful multi-core desktops (which are the primary development environment).
Processing is an open source programming language and environment for people who want to create images, animations, and interactions. Initially developed to serve as a software sketchbook and to teach fundamentals of computer programming within a visual context, Processing also has evolved into a tool for generating finished professional work. Today, there are tens of thousands of students, artists, designers, researchers, and hobbyists who use Processing for learning, prototyping, and production.
Psi is a free instant messaging application designed for the XMPP network (including Google Talk). Fast and lightweight, Psi is fully open-source and compatible with Windows, Linux, and Mac OS X.