The Magna Carta is still very relevant to today's world. It lays the foundations for the liberties and democratic process that many countries rely on today. The Magna Carta has also changed international agreements - especially in the area of human rights.
A few specific examples include:
Article 39 which states
No freeman shall be taken or imprisoned … or exiled or in any way destroyed, nor will we go upon him nor will we send upon him except upon the lawful judgement of his peers or the law of the land.
The social debate over the imprisonment of men at Guantanamo Bay (without trial) comes back to the principle of this article.
More broadly, the Magna Carta has spurred various Bills of Rights. In modern times, the United Nation's Universal Declaration of Human Rights can be traced back to the Magna Carta. Even more recently, the British Parliament passed the Human Rights Act in 1998.