A fellow by the name of Charles Hartshorne created a very interesting version of the ontological argument in his essay "The Logic of Perfection". It is in modal logic, for those of you familiar with it. Not too hard for those who aren't.

v = Or, ~ = Not, -> = Implies, <> = Possible, n = Necessary
Let P be a perfect being, i.e. God.

1. P -> nP //Anselm's Postulate, perfection cannot be contingent.
2. ~n~P //Anselm again, perfection is possible
3. nP -> P //Modal axiom
4. nP v ~nP //Principle of the excluded middle
5. ~nP -> n~nP //Becker's postulate, all modal status is necessary
6. nP v n~nP //Substitution
7. n~nP -> n~P //Modal modus tollens
8. nP v n~P //Substitution
9. nP //See number 2
10. P

Everybody got that? It takes a little knowledge of logic, but it's not too hard. The main points of contention are number 2, and the concept of necessary existence applied to a being. The second questions both whether the proof has stable underpinnings, and even if it does, what kind of being you have proved to exist. Any being that exists as the same entity in any possible reality could be argued to be trivial. As a final note, the Catholic Church used to(might still, I don't know) take it as a tennant of faith that the existence of God could be proven with the unaided reason, quite an odd thing for an article of faith in my opinion.