Major cross-platform (Windows, Mac OS X, Unix-like) GUI toolkits are available for Python:

See also

PyGObject
PyGObject provides introspection bindings for C libraries using GObject. One of these libraries is the GTK+ 3 widget set. GTK+ comes with many more widgets than Tkinter provides. An online Python GTK+ 3 Tutorial is available.
PyGTK
PyGTK provides bindings for an older version of the library, GTK+ 2. It provides an object oriented interface that is slightly higher level than the C one. There are also bindings to GNOME. An online tutorial is available.
PyQt
PyQt is a sip-wrapped binding to the Qt toolkit. Qt is an extensive C++ GUI application development framework that is available for Unix, Windows and Mac OS X. sip is a tool for generating bindings for C++ libraries as Python classes, and is specifically designed for Python.
PySide
PySide is a newer binding to the Qt toolkit, provided by Nokia. Compared to PyQt, its licensing scheme is friendlier to non-open source applications.
wxPython
wxPython is a cross-platform GUI toolkit for Python that is built around the popular wxWidgets (formerly wxWindows) C++ toolkit. It provides a native look and feel for applications on Windows, Mac OS X, and Unix systems by using each platform’s native widgets where ever possible, (GTK+ on Unix-like systems). In addition to an extensive set of widgets, wxPython provides classes for online documentation and context sensitive help, printing, HTML viewing, low-level device context drawing, drag and drop, system clipboard access, an XML-based resource format and more, including an ever growing library of user-contributed modules.

PyGTK, PyQt, and wxPython, all have a modern look and feel and more widgets than Tkinter. In addition, there are many other GUI toolkits for Python, both cross-platform, and platform-specific. See the GUI Programming page in the Python Wiki for a much more complete list, and also for links to documents where the different GUI toolkits are compared.