The Standard Widget Toolkit is a native widget toolkit for Java that was invented as part of the Eclipse project. SWT will use the running platforms widget toolkit (such as Windows API or GTK+) underneath.
Cross-platform, based on the programming languages C or C++, often with bindings to other languages:
Tk, a widget set accessed from Tcl and other high-level script languages.
GTK+, open source (LGPL), primarily for the X Window System, ported to and emulated under other platforms; used in the GNOME desktop environment
Qt, open source (GPL) under Linux / X Window, proprietary under Mac OS and MS Windows; used in KDE