Baltimore Road Runners Club - Occasional Trail Races
http://www.brrc.com/ 

NCR Trail Snails - Trial runs in Loch Raven and on the NCR trail
http://www.ncrtrailsnails.com/

Quantico Orienteering Club - train that Nav!
http://qoc.us.orienteering.org/

Mid-Atlantic Off Road Enthusiests - Mountain Biking
http://www.more-mtb.org/