Mark 16:20 (NKJV)
And they went out and preached everywhere, the Lord working with them and confirming the word through the accompanying signs. Amen.
This is the last verse of the gospel of Mark and speaks of the many miracles that the apostles performed. They were able to miraculously heal, speak other languages, raise the dead. It seems that these abilities would be useful today. But this verse explains why God allowed them to do such things. The miracles were called signs which confirmed or authenticated the word they were preaching. Where is that word today? It is in our Bible, both old and new testaments. Now that God's word is complete, it no longer needs authentication. The word itself stands as its own testimony. Of course what is the greatest miracle of all? It is the miracle of the new birth--that we can become children of God.
Hebrews 2:3-4 (NIV)
How shall we escape if we ignore such a great salvation? This salvation, which was first announced by the Lord, was confirmed to us by those who heard him. God also testified to it by signs, wonders and various miracles, and gifts of the Holy Spirit distributed according to his will.