This saying is among the most commonly misused in everyday culture, which goes something like this, "You claim to be a Christian. Even Jesus commanded you not to judge others. Who are you to judge others for their lifestyles, or to call what someone else is doing 'sin?'"
This is a classic example of Jesus' words being yanked out of context. Let's take a closer look at the passage where this saying is found: “Judge not, that you be not judged. For with the judgment you pronounce you will be judged, and with the measure you use it will be measured to you. Why do you see the speck that is in your brother’s eye, but do not notice the log that is in your own eye? Or how can you say to your brother, ‘Let me take the speck out of your eye,’ when there is the log in your own eye? You hypocrite, first take the log out of your own eye, and then you will see clearly to take the speck out of your brother’s eye." (Matthew 7:1-5, ESV)
From the larger context we discover that Jesus is condemning hypocritical judgment of the petty variety. The person doing the judging of the other one has the very same problem, only worse. And so the standard the hypocrite uses to judge someone will be the exact same standard applied to him. Jesus is saying that if someone is going to exercise judgment, he needs to take care of the very sin in his own life that he condemns in another. Once that is done, helpful, constructive criticism of the other is encouraged.
The very next verse indicates that some kind of "judging" is required in order to identify the "dogs" and the "pigs" (Matthew 7:6), and further down in the chapter Jesus warns about false prophets who can be identified by their fruits (Matthew 7:15-20). A discerning Christian practices "fruit inspecting" and so the Lord's command "not to judge" certainly does not prohibit this.
This notion that no one can say anything negative about a lifestyle or behavior based upon Jesus' command not to judge is just plain incorrect.