_ is indeed a very popular choice for “a name which doesn’t matter” — it’s a legal name, visually unobtrusive, etc. However sometimes these very qualities can hinder you. For example, the GNU gettext module for I18N and L10N, which is part of Python’s standard library, idiomatically uses _ very differently, with idioms such as…:
_ = gettext.gettext
# ...
print _('This is a translatable string.')
to mark and translate all the literal-string messages in the code (also exploiting the relative visual unobtrusiveness of _('...'). Obviously any code using this module and idiom shouldn’t also be using _ to mean something completely different (“a don’t care name”).
So a second useful alternative can be to devote the name unused to indicate such “don’t care” situations in a visually more explicit way. Google’s python style guide recommends using either _ or a prefix of unused_ — the latter can be a bit verbose but tends to be very clear, e.g.:
name, unused_surname, salutation = person_data
print "Hello, %s %s!" % (salutation, name)
makes crystal-clear that person_data is a three-item sequence (probably a tuple) and the item you’re skipping (and not using at all) is the surname (because you want to print a friendly message like “Hello, Mr Alex!” or “Hello, Miss Piggy!” ;-). (pylint and similar tools can warn you if you have unused variables named otherwise than _ or unused_..., and of course also warn you if you ever do use a variable named unused_something!-).