PUMP (Portland United Mountain Pedalers) is our local mountain bike advocacy group. They work to maintain trails and make sure the interests of mountain bikes are represented in often thorny land access issues.
PUMP was also instrumental in putting together this great (and free) map of Forest Park (PDF).
Now they’re gaining momentum for a very exciting project; The Portland Dirt Experiment (PDX, get it?) Bike Park. This would be a designated mountain biking playground where freeriders and dirt-lovers from all over the country could come and play, without fear of running hikers off the trail, or eroding sensitive habitats.