Trails.com publishes online many of the trail descriptions in the Falcon Guides trail books, such as “Mountain Biking Denver and Boulder”. We discovered the hard way that their trail ratings are unreliable. The Ceran/St. Vrain Loop up by Jamestown was much harder than they lead us to believe. Trails.com has some free information, but for access to detailed info you must pay a $4/mo. fee.
Mapmyride.com is another option. You can search for previously mapped rides or easily map and describe your own rides, which can be shared with others.
Gobikeboulder.net is helpful for city commuters to plan their routes, but it doesn’t have all the useful shortcuts that you will find on your own. It’s a good way to get started though.
Please add your comment below if you have any good resources for finding rides.