Ligonier Ministries is offering many of R.C. Sproul's audio and video teaching series online for free here. The many series available for free include his classic Biblical and theological teaching on being chosen by God, the holiness of God, the providence of God, justification by faith alone, and the Trinity.
There are also many series available on apologetic issues. You will definitely want to check out these free resources from one of this country's greatest Reformed teachers.