One of the best mapping applications that I've come across is Quo from a company called Mapyx (www.mapyx.com) The Quo application is extremely powerful and uses both OS and Open Source maps, as well as offering the ability for users to scan and calibrate their own maps. There is a plug-in which supports the import of Geocaching files, as well as numerous other facilities, such as route and track plotting, import/export, etc. The complete range of OS maps (1:25000 Explorer and 1:50000 Landranger) are available for purchase as 'tiles', which are only £1.89+VAT and £0.92+VAT each, so you only need to buy tiles for the immediate area you are interested in.
Best of all is the fact that the Quo application is completely free.
The only downside is that currently the program only runs under Windows. However, I got around that by using VirtualBox in Ubuntu and installing Quo in XP running on a virtual machine.
Highly recommended and well worth a try.