Spinoza is an interesting philosopher. I disagree with many of his conclusions, but there are others that I find ingenious. I'm not necessarily in support of his version of the ontological argument, but it's definitely something to think about.
1. Inability to exist is impotence. (Premise)
2. Ability to exist is power. (Premise)
3. If only finite entities exist, then finite entities are more powerful than an infinite entity. (From 1 and 2)
4. Finite entities are not more powerful than an infinite entity. (Premise)
5. Either an infinite entity exists or nothing at all exists. (From 3 and 4)
6. Something exists. (Premise)
7. Therefore, an infinite entity exists. (From 5 and 6) 
Premise (6) is, in fact, a posteriori. This leads many philosophers to conclude that Spinoza is here defending not an ontological argument, but a cosmological argument. I'm inclined to agree.
I can see (1) and (2) being somewhat controversial, but I also think most people would agree with both premises because of an intuition concerning the relationship between power and existence. Could something that has no power at all even exist? Moreover, isn't it a sign of power to exist necessarily than to possibly fail to exist at some time? This would also lend support to premises (3) and (4). Premise (5) is true (assuming that 1 - 4 are true) because if there were only finite entities in existence, it would be because they are more powerful than an infinite entity, which (4) states is false. As a result, it anything at all exists, it implies that an infinite entity exists. Given that something exists, per premise (6), it follows that an infinite entity (God) exists.
Where I obviously disagree with Spinoza is on his pantheism. He associates God (the infinite entity) with everything because he assumes that to be distinct from the infinite is to not exist at all, but to have any attribute in common with the infinite implies that the two entities are one and the same. The fallacy here is fairly easy to detect. Two entities can be distinct even though they share one or more attributes, since they may differ in other attributes. This is confirmed by our additional observations that some things (finite entities) come to be and pass away. They wouldn't fail to exist if they were identical with the infinite.
But, who knows? Maybe I'm oversimplifying Spinoza's view. In any case, it's something to think about.
 For a similar rendering, see: William Lane Craig, The Cosmological Argument from Plato to Leibniz, Wipf and Stock Publishers, 2001, p. 244.