Because Python is used in a wide variety of ways, it's almost impossible to give an authoritative answer to this question. In general, any application that can benefit from the inclusion of a language optimized for speed of development is a good target Python application domain. Given the ever-shrinking schedules in software development, this a very broad category.
A more specific answer is less easy to formulate. For instance, some use Python as an embedded extension language, while others use it exclusively as a standalone programming tool. And to some extent, this entire book will answer this very question -- it explores some of Python's most common roles. For now, here's a summary of some of the more common ways Python is being applied today:
Portable command-line tools, testing systems
CGI web sites, Java applets, XML, ASP, email tools
Graphical user interfaces
With APIs such as Tk, MFC, Gnome, KDE
C/C++ library front-ends, product customization
Persistent object stores, SQL database system interfaces
With client/server APIs like CORBA, COM
Throwaway or deliverable prototypes
Replacing special-purpose parsers with Python
Image processing, numeric programming, AI, etc.
Buses Considered Harmful
The PSA organization described earlier was originally formed in response to an early thread on the Python newsgroup, which posed the semiserious question: "What would happen if Guido was hit by a bus?"
These days, Guido van Rossum is still the ultimate arbiter of proposed Python changes, but Python's user base helps support the language, work on extensions, fix bugs, and so on. In fact, Python development is now a completely open process -- anyone can inspect the latest source-code files or submit patches by visiting a web site (see http://www.python.org for details).
As an open source package, Python development is really in the hands of a very large cast of developers working in concert around the world. Given Python's popularity, bus attacks seem less threatening now than they once did; of course, I can't speak for Guido.
On the other hand, Python is not really tied to any particular application area at all. For example, Python's integration support makes it useful for almost any system that can benefit from a frontend, programmable interface. In abstract terms, Python provides services that span domains. It is:
Given these general properties, Python can be applied to any area we're interested in by extending it with domain libraries, embedding it in an application, or using it all by itself. For instance, Python's role as a system tools language is due as much to its built-in interfaces to operating system services as to the language itself. In fact, because Python was built with integration in mind, it has naturally given rise to a growing library of extensions and tools, available as off-the-shelf components to Python developers. Table 1-2 names just a few; you can find more about most of these components in this book or on Python's web site.
Sockets, threads, signals, pipes, RPC calls, POSIX bindings
Graphical user interfaces
Tk, PMW, MFC, X11, wxPython, KDE, Gnome
Oracle, Sybase, PostGres, mSQL, persistence, dbm
Microsoft Windows tools
MFC, COM, ActiveX, ASP, ODBC, .NET
JPython, CGI tools, HTML/XML parsers, email tools, Zope
DCOM, CORBA, ILU, Fnorb
Other popular tools
SWIG, PIL, regular expressions, NumPy, cryptography
Part I: System Interfaces
Parallel System Tools
Larger System Examples I
Larger System Examples II
Part II: GUI Programming
Graphical User Interfaces
A Tkinter Tour, Part 1
A Tkinter Tour, Part 2
Larger GUI Examples
Part III: Internet Scripting
Larger Web Site Examples I
Larger Web Site Examples II
Advanced Internet Topics
Part IV: Assorted Topics
Databases and Persistence
Text and Language
Part V: Integration
VI: The End
Conclusion Python and the Development Cycle