Brimstone Recreation operates an offroad park located in Huntsville, TN. Their website contains contact information and directions to their office. Brimstone also offers camping and cabins near the trailheads. The park itself covers over 19,000 acres with 300+ miles of trails and is seperated into north and south sections. During our visit we only had time to see a small portion of the north side.
The trail system seems to be well marked and is categorized into 3 sections. Green for easy gravel roads, blue for moderate, and red for difficult trails. We found the categories to be fairly accurate, although we did find one trail that was labeled blue on the map that had been correctly marked at the trailhead as red. So when in doubt, go with the trailhead markers as these will obviously be more current.
The 2 red trails we drove varied in difficulty quite a bit. One (Trail 4) was rough but plenty wide enough for our Jeeps. The other trail (Trail 87) required winching in several places and was quite narrow in spots.
Overall Brimstone offers spectacular views of the surrounding countryside and well marked Jeep friendly trails. We hope to return there again soon to explore the south side as Brimstone is a really enjoyable place to wheel.
EDIT: As of October 2012 apparently Jeeps are no longer welcome at Brimstone. I would recommend calling them before you go unless you intend to ride an atv