I first heard of Tim Keller through D.A. Carson's book Becoming Conversant with the Emerging Church. Carson offered Keller as an example of someone who is reaching the emerging generation without being part of the emerging movement.
I don't know when exactly I began to listen to Keller, but I'm certainly in a phase of learning from him. He has a grasp of theology, gospel, and culture that's rare, and is a good example of an effective scholarly practitioner.
(1) Preaching to the Heart without being Legalistic
(2) Preaching to the Heart without being Piestic
(3) Unintentional Preaching Models
(4) Reading, Preparation, Conversation, and Preaching
(5) Preaching to "Emerging" Culture
(6) Preaching to the Heart without being Individualistic
The PDf files for these talks are also included on a separate CD.