Does Deuteronomy 18:21-22 still apply today? Deuteronomy 18:21-22 (NLT)   
  
  21&nbsp;“But you may wonder, ‘How will we know whether or not a prophecy is from the Lord?’ 22&nbsp;If the prophet speaks in the Lord’s name but his prediction does not happen or come true, you will know that the Lord did not give that message. That prophet has spoken without my authority and need not be feared.


Thus, the test of an Old Testament prophet was whether or not they prophesied correctly. However, I heard last weekend that modern-day prophecies are not always correct, much like when Galadriel (from Lord of the Rings) says (roughly) "Remember that the Mirror shows many things that have not come to pass, and may still not.". Given that prophesying still happens today (Acts 2:16-18, 1 Corinthians 14), I ask: does this Old-Testament test of correctness still apply today?
Whether or not prophesying (or, at least future-telling prophesying) still happens today is up for debate (see What is the basis for Cessationism).  However, if it does still happen today, then most Christians would agree that the Prophet's Test you mentioned is still very important.

Prophecy is only defined as such when it is a revelation from God.  And since it is impossible for God to lie, any so-called prophecy that proves false must not have been from God.  The principle of the passage still applies: if a prophet speaks presumptuously, asserting God's authority when God's authority was not actually given, that "prophet's" credibility is shot - he cannot be trusted.