They are all over the state, anything from extreme rock crawling to mild family runs.
Check out: TrailDamage.com
This is a great resource for Colorado Jeepers that lists many of the trails in Colorado and Moab.
Starting in Estes Park, there is the Pearson Park trail. Goes behind the Twin Sisters, and down to Meeker Park. From there, you can go for Parachute Hill, or take the Johnny Park trail back to Hwy 36. Not too tough, but nice scenery, and a lot of fun.
+1 Imogene is Great! Sloughterhouse Gulch near Bailey is a prety short drive from denver and I like that one. Check out the book Guide To Colorado Backroads and 4-wheel Drive Trails. I see it at a lot of Gas Stations and its for sure at Barnes and Noble. Awesome book.