How to restore a builtin that I overwrote by accident?

Just delete the name that is masking the builtin:

>>> set = 'oops'
>>> set
'oops'
>>> del set
>>> set
<type 'set'>

You can always still access the original built-in through the builtins module (__builtin__ on Python 2, with underscores and no s); use this if you want to override the built-in but want to defer to the original still from the override:

>>> import builtins
>>> builtins.set
<type 'set'>

If you have trouble locating where the masking name is defined, do check all namespaces from your current one up to the built-ins; see Short description of the scoping rules? for what scopes may apply to your current situation.

You can use __builtin__:

>>> import __builtin__
>>> __builtin__.set
<type 'set'>

or simply(no imports required):

>>> __builtins__.set
<type 'set'>

For Python 3:

>>> import builtins
>>> builtins.set
<class 'set'>

From docs:

CPython implementation detail: Users should not touch __builtins__; it is strictly an implementation detail. Users wanting to override values in the builtins namespace should import the __builtin__ (no ‘s’) module and modify its attributes appropriately.