Jeff Mirus has written a rather balanced and quite conditional defense of Lying for "Catholic Answers" that's worth the read. He gets the issue wrong, on the whole, but does so fairly and without animus. And that's a step in the right direction on this issue.
A major mistake is when Mirus misleads his readers when he undercuts the moral authority of the Cathechism, but other than that he quite rightly recognizes that our attempts to justify lying by using the extreme scenario, "Is is right to lie to Nazis who are looking for the Jews you're harboring?" is mere pedantics and a cover for our own moral weakness.
Beyond that, there's one or two other errors in Mirus' piece that I'd like to deal with.
From the article: "In other words, we are obliged to tell the truth, and we are also obliged to keep secrets, but there are times when the only way to keep a secret is to lie." This is blatantly false. There is never a time when the only way to keep a secret is to lie. Silence is always an option. See Jesus before the High Priest, Who remained silent until pressed, and when forced to speak told the Truth, knowing it would cost Him torture, suffering and death.
If the Nazis show up and say, "Are you harboring Jews?" and you lie and say "No," do you really think they'll pass on without searching your house? If you remain silent or say (truthfully), "You have no business knowing that," they will perhaps shoot you and you will die a martyr, and they will perhaps find the Jews you're hiding (which they would have anyway), but which is more important, life and safety or Fidelity to the Truth? Fidelity is Faith and the Truth is God. Fidelity to the Truth is simply demonstrating Faith in God. Which is more important, keeping our lives secure or being faithful to the God Who made us, even in the face of suffering and death?
Now, when Mirus argues that lying to the Nazis is choosing the "lesser of two evils", his argument is confused, for refusing to lie to the Nazis is in no way a cooperation with the evil they intend to commit. "Not lying" is not the "greater of two evils". But with this argument, he is at least admitting that telling a lie is an "evil" (a sin, though perhaps a minor one), and an "evil" that we do deliberately to avoid (in this scenario) a very bad consequence. This is exactly what happens. And in practical situations, almost everyone would do this without compunction. And, indeed, I'm certain God would forgive such a minor evil - he forgives all evils when we repent of them and turn to Him - particularly one committed to save the life of another.
But in practise we are never forced to choose the "lesser of two evils". We can always opt to choose the good, even though this choice brings suffering and the cross along with it.
It may be that only a saint could be utterly truthful at all times - but the 8th Commandment does not simply apply to perjury (as Mirus would have it), it has vast implications that Christians have always recognized. We are called to bear witness to God. And God is Truth. And you simply cannot bear witness to Truth by means of a lie.
The problem is when this Nazi scenario is used to justify other forms of lying. Pretending to be a pimp in order to ensnare someone on a sting video is not the lesser of two evils. It is a choice that one is not forced to make, and it brings no glory to God, for it conveys the attitude, "I'll do whatever it takes, for the God I serve is not Truth but victory in a given situation."