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.