Keller pastors Redeemer Presbyterian Church in Manhattan. He is known for his thoughtful sermons which attract a large and varied audience. He has lectured at Google on three occasions that I am aware of (here, here, and here), as well as at Oxford and other universities. Keller is also a prolific author. Just before Christmas New York Times writer Nicholas Kristof published an interview with Keller titled, "Am I a Christian, Pastor Keller?" I think it really good, both the questions and answers, and commend it to you.
(Note: I have blogged about Tim Keller several times. Simply google "Muddling Toward Maturity - Tim Keller" for a list of posts.)