The trails have changed somewhat in the area, there is about 12 different trails within about an hours drive from where I live:
1) Yellow River was a good trail, however, about 6 years ago the county turned one side of the road into a multi-use trail, flattened everything out, made it wide, and dumped gravel over much of it. That side is now a better waking trail than a biking trail. On the other side of the road is a decent single track, although it's short, only a few miles long.
2) Blankets Creek is a nice trail, about 15 miles of trails there. It's been a few years since I've ridden there.
3) Ft. Yargo - about 12 miles of trail, not including The Monster Mile, which is a harder section of trail added that's parallel to the gas line climb. I've ridden there lots of times, however the fees have gotten a bit ridiculous, park rangers are charging about $8 to bike there now. If you don't bike, it's $5 to get in, what's up with that?
4) Tribble Mill - Not the best marked trail, when I lived closer to that trail I biked there often with friends. It's an easy trail, Singletracks.com says it's 10 miles but we have a difficult time putting together a 6 mile loop given nothing is marked well.
5) Gainesville College - Easy short trail, about 4 miles, a good beginner trail in the area.
6) Georgia Ag Center - Closed about 8 years ago.
7) Big Creek Roswell - Only rode here a few times, 6 miles but just too rocky and not very fun in my opinion.
8) Charleston Park - A newer trail, only 6 miles but is supposed to have nice views of Lake Lanier. Have not ridden there.
9) Sope Creek - 9 miles of trail, never ridden there.
10) Georgia Horse Park - 8 miles of trails, I've ridden there a few times. This trail was built for the 1996 Summer Olympics, one loop is really difficult and goes across lots of scary (IMO), smooth granite, the other loop is a decent trail, I've ridden it mostly on demo days when the Southeastern Bike Expo is in town.
11) Bull/Jake Mountain - 30 miles of trail outside the North Georgia mountains in Dalonega, I need to go there, I hear it's a very good trail.
12) Last but not least, Chicopee Woods - Now up to 26 miles of trails. This is the trail closest to where I live, only 15 min away. A good mix of mostly intermediate trails. A very good trails system, a good workout and it has some really fun fast sections.