Tim Keller has had a great impact on my thinking in the last several years. Professor turned church planter, he has a deep understanding of the Scriptures and sincere heart for the people of God. The Prodigal God is one of the best books I read in the last couple years. He was asked to give a talk on the big ideas of it at the Willow Creek Leadership Summit in August.
Keller is the Sr. Pastor of Redeemer Presbyterian Church (located in New York) also has 150 of his sermons available from free download here.