Quo mapping software FREE, plus very reasonable prices for OS Digital mapping. 1:50,000 - 99p and 1:25,000 - £1.95. Register and download the mapping software and get a further 10% off.
Quo is really good for anyone who does any outdoor activity requiring maps. I am very impressed with it. Plot routes, print out maps, connect to Google Earth to view a route, download other peoples routes form web sites and plot them directly onto OS Maps.
Best of all you can transfer routes and maps to a dedicated GPS device, or alternatively you can use a Windows Mobile based PDA with external Bluetooth GPS. (£17 if you were quick enough for the Bluetooth GPS I posted with Play last week).