And, just for fun, because it sort of breaks your brain (although I hate, hate, hate the argument personally):
Assume that God is that of which no greater being can be conceived. If this is God, then consequently, God must be able to be conceived of, yes? Well then:
(1) Suppose that God exists in the understanding alone.
(2) Given our definition, this means that a being than which none greater can be conceived exists in the understanding alone.
(3) But this being can be conceived to exist in reality. That is, we can conceive of a circumstance in which theism is true, even if we do not believe that it actually obtains (that is, that it is actual, rather than merely potential).
(4) But it is greater for a thing to exist in reality than for it to exist in the understanding alone.
(5) Hence we seem forced to conclude that a being than which none greater can be conceived can be conceived to be greater than it is.
(6) But that is absurd.
(7) So (1) must be false. God must exist in reality as well as in the understanding.
Not necessarily anything that's going to convince anyone that God exists, but a fun brain-twister.