Pidgin is a GTK+ instant messaging application for Windows and Unix. It supports AIM, ICQ, Jabber/XMPP, MSN, Yahoo!, Bonjour, Gadu-Gadu, IRC, QQ, SILC, SIMPLE and more. See http://pidgin.im/about for more information.
Wireshark is a powerful network protocol analyzer developed by an international team of networking experts. It runs on UNIX, OS X and Windows. (Looking for Ethereal? You're in the right place. We switched names in May 2006 due to trademark issues.)
Stellarium renders 3D photo-realistic skies in real time with OpenGL. It displays stars, constellations, planets, nebulas and others things like ground, landscape, atmosphere, etc.
StarDict is a Cross-Platform and international dictionary written in Gtk2. It has powerful features such as "Glob-style pattern matching", "Scan seletion word", "Fuzzy search", "Net dict", "Full text translation" etc.
ScummVM is a cross-platform interpreter for several point-and-click adventure engines. This includes all SCUMM-based adventures by LucasArts, Simon the Sorcerer 1&2 by AdventureSoft, Beneath a Steel Sky and Broken Sword 1&2 by Revolution, and many more.
Wine is an Open Source implementation of the Windows API on top of X and Unix.
Wine provides both a development toolkit for porting Windows sources to Unix and a program loader, allowing many unmodified Windows binaries to run on x86-based Unixes.
Firebird RDBMS offers many ANSI SQL features & runs on Linux, Windows & several Unix platforms. Features excellent concurrency & performance & powerful stored procedure & trigger language. WEBSITE link above right, TRACKER http://tracker.firebirdsql.org
The CvsGui project is providing several high-end interface clients (WinCvs, MacCvs, gCvs) written in C++ using popular frameworks (MFC, PowerPlant, GTK+) with a scripting support via Python or TCL.