HG foo

.hgrc

[extensions]
foo = /path/to/foo.py

foo.py

# Thanks to http://aru-and-dhi.blogspot.jp/2009/12/mercurial.html
from mercurial import hg

def foo(ui, node, **opts):
    """ Say foo! """
    ui.write("Fooooooo!!!\n")

cmdtable={
    "foo": (foo, [], "")
}

Usage

$ mkdir foo
$ cd foo
$ hg init
$ hg foo
Fooooooo!!!