There is an ever expanding number of recordings of Dharma talks available on the web. Here are a few sources that I've found helpful.
40 Days of Insight Meditation Instructions and Talks about On-Going Practice - Talks by Philip Jones for the 2010 Winter Feast for the Soul.
Abhayagiri Monastery - Talks by Ajahns Passano and Amaro, as well as others.
Audio Dharma - Talks by Gil Fronsdal and many other teachers.
Bhavana Society - Talks by Bhante Gunaratana and Bhante Rahula.
Bhikkhu Bodhi - Dharma talks by Bhikkhu Bodhi. For his talks on the Middle Length Discourses, see our sutta study page.
Dharma Seed - Talks by teachers from the Insight Meditation Society, Spirit Rock Meditation Center and others.
Forest Sangha Dhamma - Talks by Ajahn Chah and many members of his lineage including Ajahns Sumedho, Brahmavamso and Succito.
Thanissaro Bhikkhu - Talks by Thanissaro Bhikkhu on various topics.