Also excellent.

It actually makes python a full-blown shell (which python REPL is not):

    $ ipython
    Leopard libedit detected.
    Python 2.6.1 (r261:67515, Jun 24 2010, 21:47:49) 
    Type "copyright", "credits" or "license" for more information.
    
    IPython 0.10.1 -- An enhanced Interactive Python.
    ?         -> Introduction and overview of IPython's features.
    %quickref -> Quick reference.
    help      -> Python's own help system.
    object?   -> Details about 'object'. ?object also works, ?? prints more.
    
    In [1]: ls
    Applications/ Downloads/    Movies/       Public/       Workspace/
    Desktop/      Dropbox/      Music/        Sandbox/
    Documents/    Library/      Pictures/     Sites/
    
    In [2]: ls | grep App
    Applications/
    
    In [3]:

The advantages of using a REPL versus just coding is that you can test some things out before writing the code. To be honest, I don't use it much while programming, but it's indispensable for doing some Django queries or for playing with live code (you can run it in the shell and break to inspect the stack).

Its advantage over the stock Python repl is that it has better code completion, colours, pretty printing, nice history, built-in profiling and many other niceties I enjoy.