Muslims are allowed to lie to unbelievers in order to defeat them.
Taqiyya - Saying something that isn't true as it relates to the Muslim identity.
Kitman - Lying by omission. An example would be when Muslim apologists quote only a fragment of verse 5:32 (that if anyone kills "it shall be as if he had killed all mankind") while neglecting to mention that the rest of the verse (and the next) mandate murder in undefined cases of "corruption" and "mischief."
Tawriya - Intentionally creating a false impression.
Muruna - 'Blending in' by setting aside some practices of Islam or Sharia in order to advance others.
Though not called Taqiyya by name, Muhammad clearly used deception when he signed a 10-year treaty with the Meccans that allowed him access to their city while he secretly prepared his own forces for a takeover. The unsuspecting residents were conquered in easy fashion after he broke the treaty two years later. Some of the people in the city who had trusted him at his word were executed.
Another example of lying is when Muhammad used deception to trick his personal enemies into letting down their guard and exposing themselves to slaughter by pretending to seek peace. This happened in the case of Ka'b bin al-Ashraf (as previously noted) and again later against Usayr ibn Zarim, a surviving leader of the Banu Nadir tribe, which had been evicted from their home in Medina by the Muslims.
At the time, Usayr ibn Zarim was attempting to gather an armed force against the Muslims from among a tribe allied with the Quraish (against which Muhammad had already declared war). Muhammad's "emissaries" went to ibn Zarim and persuaded him to leave his safe haven on the pretext of meeting with the prophet of Islam in Medina to discuss peace. Once vulnerable, the leader and his thirty companions were massacred by the Muslims with ease, probably because they were unarmed, having been given a guarantee of safe passage (Ibn Ishaq 981).
Such was the reputation of Muslims for lying and then killing that even those who "accepted Islam" did not feel entirely safe. Consider the fate of the Jadhima. When Muslim "missionaries" approached their tribe, one of the members insisted that they would be slaughtered even though they had already "converted" to Islam to avoid just such a demise. However, the others believed they could trust the Muslim leader's promise that they would not be harmed if they simply offered no resistance. (After convincing the skeptic to lay down his arms, the unarmed men of the tribe were quickly tied up and beheaded - Ibn Ishaq 834 & 837).
Today's Muslims often try to justify Muhammad's murder of poets and others who criticized him at Medina by falsely claiming that they broke a treaty with their actions. Yet, these same apologists place little value on treaties broken by Muslims. From Muhammad to Saddam Hussein, promises made to non-Muslim are distinctly non-binding in the Muslim mindset.