tried a few – quite like everytrail
there is a free version, and one which costs £2-3.
The paid version lets you download other people’s trail onto your map which is handy for finding ‘secret’ singletracks.
But I mostly ride Surrey Hills and for that area it has all the well known trails and quite a few others marked on one of the maps (terrain?) because it uses that wiki open source bike map thing – that I cant remember the hame of.
EDIT one thing I would add is that the assisted GPS on the iphone is nowhere near as accurate as on a Garmin. I still use my Garmin for proper tracking but everytrail is good for finding your way round.
So say you’d never been to Whiteways on the S Downs you could download this map http://www.everytrail.com/view_trip.php?trip_id=948604 onto your iphone and then you would be able to find the Denture trail pretty easily.