Urban Areas :
Parkwood Springs -
Good grass area (2x football pitches, +1 further back) but can be busy at the weekends with matches. But there are are walking trails and downhill tracks around the back which are good. Would be better for smaller than 5th scale as corners are tight.
Don Valley Bowl -
Cricket pitch size green, but with amphitheatre stepped back. So long as there is not an event there its normally quiet and good for some jumps.
Wooly Wood BMX -
Few too many kids on nice days/holidays, but occasionally its a great spot as the BMX track is more humps than jumps, so a 5th scale doesn't get too much height as to destroy it.
There is some woodland behind and then you can get to Concorde Park for some open grass, but you need to cross the golf course first.
Other Area :
Grenoside Woods -
A bit of a car killer. I've hit too many trees at 30mph+ which ends your day fast. But if you go to the more open areas there is still some fun to be had. Again, might be good for the smaller scales. There is a large downhill mountain bike course there (Steve Petes downhill course)
Tankersley Motocross -
At Tankersley round-a-bout there used to be a moto-cross course with some other areas. This used to be a great place, but I hear that it might have now all been dug up and turned into a shooting range, but I have not been in over a year.
Old Thorncliffe Colliery -
Huge open spaces of dirt/very fine gravel interspersed with some groups of trees and raised areas. This is one of my favourite areas as you hardly ever see anyone there and its huge. Only 2 downsides... Its where the old open cast mine used to be and you are bashing on the old slag heap, this means that you are about 4 meters above ground level and about 3-5 minute walk from your car - if its wet, expect mud, lots of mud. Over winter I found that the rain had caused some major gullies which are had to see from a distance - hit one of those at full tilt and its going to break something... So always best to reccy the area first.