Thunderbird is a total redesign of the Mozilla mail component to produce a cross-platform, stand-alone mail application using the XUL user interface language. It has many new features, among them the ability to customize your toolbars the way you want them. a new look and feel with a large number of downloadable themes which alter the appearance of the client, and the ability to add UI extensions.
GroupOffice is a groupware suite that takes your office online, implementing online collaboration and CRM. It allows you to share projects, calendars, files and email online with co-workers and clients. It is easy to use and fully customizable. It also features synchronization with PDAs and Outlook.
XWiki is a WikiWiki clone written in Java that supports many popular features of other Wikis like the Wiki syntax, version control, attachments, security, and searching, but also many advanced features like templates, database and dynamic development using scripting languages (Velocity, Groovy, Ruby, Python, PHP, and more), an extension system and skinability, J2EE scalability, an XML/RPC remote API, statistics, RSS feeds, PDF exporting, WYSIWYG editing, an Office viewer and importer, and a lot more.
FF Multi Converter is a simple graphical application that converts audio, video, image, and document files between all popular formats, using and combining other programs. It uses ffmpeg for audio/video files, unoconv for document files, and the ImageMagick library for image file conversions.
4Pane is a four pane detailed-list file manager for Linux. It is designed to be fully-featured without bloat, and aims for speed rather than visual effects. In addition to standard file manager things, it offers multiple undo and redo of most operations (including deletions), archive management including 'virtual browsing' inside archives, multiple renaming/duplication of files, a terminal emulator, and user-defined tools.