A huge area of smooth rock and canyons right on the edge of town. The trails were originally developed by motorcycle riders in 1969 and are now open to mountain bikes also.  A white dotted line shows the way and the the trail has several very steep climbs and descents but the rock is incredibly grippy so there's nothing that's impossible.  There were some nice views of the Colorado River, although it was quite overcast and not good for pics.

One of the most well-known trails in Moab that leads to the infamous Portal trail (a rider went over a 300' drop last year).  Unfortunately it seems to be a jeep trail and not too interesting for mountain bikes.  Lots of soft sand that's impossible to ride through and lots of rocks and slickrock that might be fun in a jeep but it didn't do much for me.  I turned back before the Portal Trail so perhaps it's that part that gives it its reputation (although the guys in Moab Cyclery said they didn't like Poison Spider either).