I recently picked up a copy of Plato's Republic (OK, two actually), and at first glance, Plato's just and unjust is not unlike the Buddha's distinction between skillful and unskillful actions (kamma). Both seem like a middle way between, or possibly a synthesis of, Jeremy Bentham's teleological utilitarianism and Immanuel Kant's deontological categorical imperative.
That's not to say that Bentham and Kant represent two ends of a single ethical spectrum, only that Plato and the Buddha take what Bentham and Kant stress and emphasis them together. With Plato and the Buddha, just/skillful actions aren't simply judged to be just/skillful based upon their consequences, but also because there's something inherently just/skillful about the actions themselves. In Buddhism, this would be due to the quality of the intentions behind the actions, and I think a similar principle applies in the Republic as well, although Plato would obviously say that it's because they share in the form of Justice, or even of the Good.
But despite the similarities, the Buddha does place far more emphasis on the consequential aspect of actions than Plato in determining whether they're just/skillful (e.g., MN 61), so ethically speaking, he falls decidedly more into Bentham's camp than Plato.
I suspect that this might be a consequence of Plato's denial/rejection of empiricism, which, in the Republic, forces him away from a more consequentialist position until Book 10. Up until then, he appears to limit himself to what Stephan Watt terms 'natural consequences,' and not ones "dependent on other people's recognising and rewarding your justice." Whereas the Buddha accepts a form of empiricism that's, in the words of David Kaluphana, "based broadly on ordinary sense experience and extrasensory perception" (Buddhist Philosophy: A Historical Analysis), Plato seems to take great pains to avoid using empirically-based observations to support his propositions for as long as possible. However, he's still compelled to utilize them at some point in many of his arguments.
Perhaps this is simply for the benefit of his less philosophically advanced interlocutors, but his rejection of sense-data as a reliable basis for knowledge seems ironic when many of his arguments are themselves based on sense-data. (I'm mainly thinking of the Phaedo here, where Socrates is arguing for the immortality of the soul and supporting his propositions with empirical observations, or where he does the same thing in explaining causation using his theory of forms.) Or maybe I'm just misunderstanding Plato since he seems like such a smart guy and it's hard for me to believe that he really failed to spot this glaring epistemological dilemma in his philosophy.
Nevertheless, both Plato and the Buddha seem to agree that acting justly/skillfully leads to being a better person, a morally superior type of individual both wise and at peace, someone possessing a noble and unshakable character. For the Buddha, who I find much more direct than Plato, skillful actions, when used appropriately, have the potential to ultimately lead to the elimination of the skillful/unskillful dichotomy altogether, leaving only moral perfection (i.e., the end of kamma). But Plato uses the just city as a backdrop to illustrate his just individual, and I'm not quite sure exactly how much is allegorical.
In any case, it's not hard to compare what's in the Republic to many of the things the Buddha is recorded as saying in the Pali Canon considering the wide variety of similar themes and ideas scattered throughout. For example, Plato's comment at the beginning of 546a, which itself may have been derived from Heraclitus, mirrors the Buddha's maxim regarding origination and cessation at SN 56.11. Then there's 571d-572b, which sounds similar to the Buddha's advice regarding the practice of virtue, restraint and moderation and the cultivation of meditation and discernment in an effort to 'touch the deathless' (amata). (I find similar parallels between Socrates' dialogue in Plato's Symposium regarding immortality and the idea of forms and the Buddha's discourses regarding the deathless, as well.)
The beginning of Plato's third argument supporting his conclusion that the happiest and most just individual is the one who rules over him/herself like a king (with virtue, moderation and reason) sounds Buddhist-esque with the 'motion' of pleasure and pain in the soul bringing the teachings on kamma to mind, while the 'state of calm' intermediate between pleasure and pain is equivalent to Buddhist equanimity (upekkha). And while for Plato the soul is something immortal that reincarnates, he gives an argument later on in Book 10 (612e-613b) that resembles parts of the Buddha's teachings on kamma and rebirth in MN 136.
Even more interesting, however, is 582b-d, where "that which is related to what is always the same, immortal and true" and "that which is related to what is never the same and mortal" are akin to nibbana and samsara respectively. The allegory of the cave at the beginning of Book 7 immediately comes to mind here, where Plato uses the image of the philosopher turning the soul (mind) away from the darkness of the visible realm (samsara) towards the light of the form of the Good (nibbana).
Of course, there are certainly things I disagree with Plato about — such as what sounds like the 'social Darwinism' of Herbert Spencer in 407d-408b and his manipulative and uber-utilitarian use of religion and tradition to mask his imaginary social engineering throughout 459d-461e — and I’m not under any illusions that their messages are identical.
Overall, I can't help thinking Plato makes an unnecessary, and ultimately artificial, distinction between opinion/becoming and intellect/being, which is another result of his reliance on rationalism to the exclusion of empiricism. While I agree that sense perceptions can be misleading, I think it's a mistake to deny them altogether, especially if, like the Buddha, we include mind itself as a sense-base. If what our eyes see is always a shadow, an illusion, how can we be sure that the same isn't true of the mind's intellections? I don't think Plato sufficiently deals with this problem as he seems to hold the mind and its intellection of intelligible things sacrosanct.
For Plato, to truly know a thing, you must know its essence or being, which is something intelligible but not perceptible via the senses. For example, in 596e-597a, he treats the form or idea of 'bed' (being/essence) as something more real and true than an actual bed made by hand (appearance). In other words, the many things we experience and consider to be true are in reality imperfect representations of 'that which is' (forms). And when Plato does attempt to address this issue, his arguments often extend, ironically enough, from more or less empirical observations. Or even worse, from abstract assumptions that are seemingly taken for granted and rarely accounted for, such as the very existence of forms.
However, I do think that, in some regards, Plato and the Buddha are ultimately saying something very similar; although to me, the Buddha seems more confident, like he's pointing you towards the experience instead of trying to stumble upon it through reason alone. Plato, on the other hand, seems like he's coming at it from a more theoretical, less experiential point of view, almost as if he's unsure it's completely possible.
It could just be that I'm reading too much into his politics thanks to Watt's introduction, which has me seeing the Republic in terms of psychology as well; but the more I read it, the more I find myself moving away from seeing the Republic in terms of a dialogue about social engineering and towards something more, for lack of a better word, Dhammic.