IPython

?
%quickref
object?
object??

%history
%timeit [math.sin(x) for x in range(500)]
<TAB>                       # autocompletion
files  = !ls                # store output of a command in a variable

%who                        # show vars in scope
%whos                       # detto, detailed.

%pdef os.path.join
%pdos os.path.join

_                           # previous output (just like default python)
_i                          # previous input

%doctest_mode

*l?                         # show all variables ending with `l`