[Boromir88]

Great job alatar and William in bringing this topic up.

Tolkien was at great pains to avoid a Manichaean world.~William

For the most part I agree...although I think The One Ring is a different matter.

Tom Shippey makes the argument that Tolkien encorporates both Boethian and Manichaen types of evil in The One Ring. Although this has gone contested by others, I think Shippey's got a point.

The Ring has a lust and power that actually exudes evil itself:

'so great was the Ring's power of lust, that anyone who used it became mastered by it...'~Letter 131

This is Tolkien's Letter to Milton Waldman, and he makes the point that the Ring has it's own innate evil that it exudes on people. There is further mentioning in Letter 246:

'It was part of the essential deceit of the Ring to fill minds with imaginations of supreme power.'

Also from Letter 246, so great was the Ring's power (its influence would reach a 'maximum' when brought into the Sammath Naur) that it was impossible for Frodo (or anyone to destroy):

'I do not think that Frodo’s was a moral failure. At the last moment the pressure of the Ring would reach its maximum - impossible, I should have said, for any one to resist, certainly after long possession, months of increasing torment, and when starved and exhausted.'

With that being said when dealing with the Ring, it is not purely Manichaen. If it was I think The Ring would be this all-corrupting force that just corrupts everyone that's around. I think what Gandalf says points to the duality of evil 'within' (Boethian) and evil without (Manichaen):

'Do not tempt me! For I do not wish to become like the Dark Lord himself. Yet the way of the Ring to my heart is by pity, pity for weakness and the desire of strength to do good. Do not tempt me!...'~The Shadow of the Past

The Ring would not only be able to use Gandalf's 'positive purposes' (pity and a desire to do good), to turn him into virtually another Sauron. But we see here Gandalf's rejection of the Ring ('Do not tempt me!'). Also, there were several others who rejected the Ring (Sam, Galadriel, Bilbo), and I don't think I can put it any better than Faramir's words to Frodo:

'But fear no more! I would not take this thing, if it lay by the highway. Not were Minas Tirith falling in ruin and I alone could save her, so, using the weapon of the Dark Lord for her good and my glory. No, I do not wish for such triumphs, Frodo son of Drogo.'~The Window on the West

So, there is also the factor of dealing within the person. Or how would characters like Gandalf and Faramir be able to reject the Ring, yet others (like Gollum and Boromir) fall to it?

I don't know about any other examples but with The One Ring I do think we see a duality of the two aspects of evil. Not only does the Ring exude evil itself on to others, but it is Boromir and Gollum's predisposition to greed, glory, and power, from within themselves which led to their corruption by the Ring. While others who did not desire such 'triumphs' were able to reject it.