Python2 implicitly converts int to long and str to unicode:

    >>> 2**100
    1267650600228229401496703205376L
    >>> 'x' + u'y'
    u'xy'

All Pythons implicitly convert int to float and int or float to complex:

    >>> 1 + .5
    1.5
    >>> 2 * 3j
    6j

Methods like list.extend now, in recent versions of Python (since 2.1), accept arbitrary iterables rather than just lists; it's more debatable whether this is an “implicit type conversion” or not.

    >>> x = [3, 4]; x.extend((5, 6)); x
    [3, 4, 5, 6]