John Wilkins has a nice post on design at "Evolving Thoughts." One thing I disagree with quite a bit, though, is the attribution of a design argument to Aquinas. Wilkins points to the Fifth Way (ST 1.2.3), quoting a common translation:
...things which lack intelligence, such as natural bodies, act for an end, and this is evident from their acting always, or nearly always, in the same way, so as to obtain the best result. Hence it is plain that not fortuitously, but designedly, do they achieve their end.
The problem is that the Fifth Way is not actually a design argument. The phrase translated by 'designedly' here is actually ex intentione; but ex intentione does not signify design but orientation. The Fifth Way is actually an argument not from design but from the fact that there's any causation at all. On Aquinas's scholastic adaptation of Aristotle, the end or final cause is what selects the effect for the efficient cause -- in other words, it is what answers the question, "Why does this cause produce this effect rather than some other effect?" The disposition of the cause to the end is its intentio. The word is associated in the medieval imagination with archery: the aim of the arrow is its intentio. So the argument of the Fifth Way is roughly that because nonintelligent things act regularly in order to achieve an end, they must achieve their end not a casu, by chance, but ex intentione, by being disposed to it. But things not capable of determining their own ends have to be, in the end, disposed to them by things capable of determining ends, namely, intelligences. So what is supposed to be at stake in this argument is not design but any sort of causation that is not due to deliberate self-determination; what's being examined is the very possibility of bodies having effects at all. This is perfectly general; final causes are for Aquinas the explanation for the fact that efficient causation occurs at all.
Aquinas does make design arguments, but he makes them in the way they usually were made before the early modern period, i.e., as arguments not about God's existence but about God's activity: given that God exists, why would we think that God interacts providentially with the world? (Compare Leibniz's argument in the Discourse on Metaphysics that, given that we know that God exists, it's not reasonable to ignore Him as a possible explanation for certain types of order in the world.) A more plausible locus classicus for design arguments as we usually think of them is the discussion of contrivance in Paley or, slightly earlier, Boyle's essay on final causes, Newton's Optics, or Hume's Dialogues on Natural Religion. But this disagreement aside, it's a post worth reading.