MP3 Shiur - Digital Torah
Shiurim in Halacha, Gemorah and some general topics.
TeachItToMe.com - Live and MP3 Torah Classes
Live shiurim on various topics, some archived complete series.
Think Jewish International
An extensive assortment of online (real audio) courses, including studies of the talks of the Lubavitcher Rebbe.
Yeshiva Live
Live broadcast of Chabad classes, informal gatherings, personal interviews, and a growing online library of written, audio and video content