Jitsi is a cross-platform and Open Source instant messaging and video/audio calling application written in Java.
Kiwix is an offline reader for offline content. It allows you to download the whole Wikipedia for offline reading.
Great Little Radio Player is a desktop application that lets you play online radio stations without using a web browser.
Cortex is an Ncurses application that lets you browse Reddit directly from the terminal.
jTimeSched is a small Java application that allows to easily track your tasks/projects by recording how much time you have spent on each task.