Memory Map is good. Plot your route on 1:2500 mapping before or during a ride. Download the map for the area you will be riding in and won't need to rely on a signal either.
OS also does free UK mapping but it is on the slow side.
Trails is a good tool for recording your rides.
Aroundme is a good app for finding banks, petrol stations, cafes etc when away from home.
Photobucket, flickr and eBay all have handy apps for the RetroBiker.
Sneezies is a great game for keeping the kids quiet on Sunday mornings whilst you get an extra tem minutes kipp.