Fairmount Park. Valley Green area along the Wissahickon Creek (Permit required)

Pennypack Park. many starting points, but most like Pine Rd parking lot. Various trails, also great for new people.

Belmont Plateau. My favorite spot! used for MTB racing, packed dirt,mud, parachute hill (big!) collarbone creek, and the 'twisty windy' trails. You can spend a whole day in there! I intro my 6yr old nephew there. You can customise your ride here, if you don't want to climb, just take connecting trail, they all criss cross.

Morris Park. 68th and Landsdowne Ave. My second fav spot! quiet, hilly, Santa visits every Xmas (REALLY, He posed on my RED bike!! - I got pictures:-) Old rock quarry, its a quick ride in itself, a 30 foot wide creek to cross (optional) really test your skills, 3 blocks away is the north side of Cobbs Creek Park, once you find a trail into it, you may find your stolen car! nice trails, get to see a waterfall that rivals Niagara Falls, really pretty in Fall, mud, dirt, gravel. You can also spend 4 hours riding around. Great stores to go eating.

Cobbs Creek South of Market St. Flat like river drives but not as much fun (no flames please) but good for bringing kids.

The permit requirements have changed. Please check these links
to see if you are required to have a permit before you ride

Trail system page located:
http://www.phila.gov/fairpark/recreation/trailsystem.html
Trail regulation page located at:
http://www.phila.gov/fairpark/recreation/trailpermits.html
Current trail permit application located here:
http://www.phila.gov/fairpark/pdfs/FPC_Trail_app.pdf